It's not a straw man unless you want to claim:
That you are not a human being or...
That you do not read Scripture infallibly or...
That the Holy Spirit dictates to you, word for word, what the true meaning of Scripture is.
It's just that simple.
This is not a remarkable fact because since there wasn't any "IM" to begin with obviously there couldn't be any to guide the first Christians. But there was Christ, and after he left, there were His Apostles.
You can post paragraphs upon paragraphs in reply but the fact remains that there are no instances in Scripture where you can point to and say, "Look, here's someone just reading Scripture alone and coming to know things of God!"
No, what is seen in each instance is someone either listening to and accepting an authority figure (Jesus, one of the Apostles, or through divine revelation the truth is known) who either also uses Scripture and reason to support his claims or just uses reason to do so.
So if anything, a foreshadowing of a needed "assured infallible" authority is shown in the OT, and the beginnings of it is shown in the new. There's a significant difference between the OT and the NT that must be considered: in the OT, God had not entirely revealed Himself to man (as much as He has since Jesus' time on Earth). So we can't say, "oh, we should just rely upon the same kind of learning experiences that are found in the OT".
Also, similarly to the events in the NT before Jesus' ascension: we can't just say, "oh, I'm going to rely on having the same kind of learning experience while Jesus was on Earth" for the obvious fact that Jesus isn't here in His own individual human body.
The experiences we share in common with anyone in Scripture are those of the Apostles and their disciples after Pentecost.
Now that God has revealed Himself as much as He wills to all Mankind (no more public universal revelation to come) we are left with either relying on the same kind of revelation in the OT (but that's not going to come anymore as again, no more public revelation is forthcoming) or relying upon physical manifestations of Christ to guide our knowledge (but he has ascended to heaven) or relying upon an authority figure here to help guide us to knowkedge of the things of God. An authority figure who is himself (or itself) guided by God.
Or we are left with nothing OR we are left with a belief that God (the Holy Spirit) somehow guides each of us individually, apart from any "IM", the exact manifestation of this guidance remains a mystery.
However if this last possibility is the only possibility then we each claim to have the gift of infallibility (from the Holy Spirit) UNLESS we claim that it's not us who are infallible but He who is infallible in us.
That is the same thing as saying He talks to us individually, with either an audible voice or one inside our head, to "teach us all things". However that's self deception, which anyone can see if one is humble enough.
There is a significant difference between:
claiming Party A helps Party B with the aid of the Holy Spirit vs..
Claiming the Holy Spirit helps both party A and B directly.
The former protects Party B from error on only one given point at a time. It is not assured in the former that either Party A or.B will be correct about everything, all the time. In fact it may at some other point in time be the Holy Spirit's will that Party B teaches Party A. This is the Catholic claim.
The latter protects both from error on all points. The latter is the Protestant claim, however is by definition "infallibility" because for all things both parties are protected from error. That's the definition of "infallibility".
The Holy Scriptures cannot be infallibly interpreted by any human authority today... then it follows that each term is there for a syllogistic purpose, to link primary terms over an appropriate number of joining terms (middle terms). This is somewhat problematic for the above premise because it is an overly complex aggregation of ideas. Where are the boundaries on the primary and middle terms? I don't know. But let's take the premise at its word and assume we are not talking about private individuals per se but only that which fully satisfies all the (best guess) terms, because I think we can agree that not all interpreters of Scripture fall into the category of "human authority." I would further narrow this as stating that we are not talking about individual authority figures interpreting the text in a personal or private setting, but those interpreting the text in some official capacity, acting as an authority, and therefore merging the question of fallibility/infallibility with their exercise of authority. Else why include the qualifier "authority?"
It is indeed a straw man because of your definition of what infallibly interpreting Scripture means. The author of your argument equates it to knowing even historical, mathematical, and scientific facts beyond speculation." beyond speculation," and thus in order to be consistent with your polemicist, then you must hold that we reject any human authority from knowing beyond speculation that even the most basic commandments cannot be correctly understood without speculation. In addition to disallowing anyone from correctly understanding even natural revelation testifies to a Creator.
And by holding that this means that the Holy Spirit dictates to me word for word, what the true meaning of Scripture is, then you are going beyond what even Rome holds the IM does.
It's just that simple.
What you need to establish is the necessity of ensured perpetual magisterial infallibility as per Rome, but instead you also wrongly argued for the involvement of humans in ascertaining veracity of Truth claims, but which does not translate into an IM.
You also wrongly relegate the discernment of Truth to meaning "just me and the Holy Spirit" if one rejects the premise of an IM, but which is a false dilemma, as this does not mean rejection of the magisterial office.
Nor does Christ being an infallible authority support the necessity of Rome's IM to know Truth, as souls knew both men and writings of God were so without one, while the infallible nature of Christ and the writers of Holy Writ was under the full inspiration of the Holy Spirit, which is not the case with the IM. Now instead of trying to play games with fallacious syllogisms,