Now do you know exactly what the SSPX was commanded to do? Are you privy to the secret “doctrinal” preamble? If not, drop the nonsense about sin; because it is also a sin to bare false witness by signing a pledge you cannot honor as a Catholic.
Let the preamble first be made public and then make your judgement about who’s sinning.
Speaking of nonsense, your comment fits the bill exactly.
Do you deny that the traditional Catholic doctrine is that your superiors should be obeyed to the best of your ability unless they command you to sin?
If so, may I suggest that you read the breviary reading (in the new breviary) for the feast of St. Maximilian Kolbe. (Whether you recognize his canonization or not is irrelevant; he was an honored Franciscan religious superior who was martyred well before Vatican II and never said the new Mass. If he's not a Catholic, then neither is anyone in SSPX.)
If not, then how is what I wrote "nonsense"?
because it is also a sin to bare false witness by signing a pledge you cannot honor as a Catholic.
And the only valid reason(s) you could give for "not being able to honor" a pledge demanded by your superiors is (a) I am not reasonably able to do so; or (b) the pledge is outside the superior's area of competence; or (c) the pledge commands me to sin.
If the "doctrinal preamble" demanded adherence to particular doctrines, then cases (a) and (b) do not apply. That leaves only case (c).
We're exactly back where we started. Do you seriously want to claim that the Roman Pontiff is commanding SSPX to commit sins by agreeing with him?