There’s a difference between The “Antichrist” as in an individual’s name, and of the “Antichrist” used more like a noun - maybe due to the inherent difficulty in precise translation- but is more apparent in context where Paul’s description is used as a test of who or what is Antichrist.
Not just the pope of that time, but his adherents too, are labeled the “kingdom of the Antichrist” for setting the Pope up as a god who shall not be questioned, people who put those who disagree or question to death, ... and for failing to teach Christ paid for our sins in full.
We’re all, including Luther and all Popes, beggars before God, with nothing to offer to justify our own salvation, except for what Christ himself gave us by paying our debts on the cross.
Anyway, the context:
“The Levitical high priest was the chief priest by divine right, and yet godless high priests were not to be obeyed, as Jeremiah and other prophets dissented from the high priests, the apostles dissented from Caiaphas and did not have to obey them.
39] Now, it is manifest that the Roman pontiffs, with their adherents, defend [and practice] godless doctrines and godless services. And the marks [all the vices] of Antichrist plainly agree with the kingdom of the Pope and his adherents. For Paul, in describing Antichrist to the Thessalonians, calls him 2 Thess. 2:3-4: an adversary of Christ, who opposeth and exalteth himself above all that is called God or that is worshiped, so that he as God sitteth in the temple of God. He speaks therefore of one ruling in the Church, not of heathen kings, and he calls this one the adversary of Christ, because he will devise doctrine conflicting with the Gospel, and will assume to himself divine authority.
40] Moreover, it is manifest, in the first place, that the Pope rules in the Church, and by the pretext of ecclesiastical authority and of the ministry has established for himself this kingdom. For he assigns as a pretext these words: I will give to thee the keys. Secondly, the doctrine of the Pope conflicts in many ways with the Gospel, and [thirdly] the Pope assumes to himself divine authority in a threefold manner. First, because he takes to himself the right to change the doctrine of Christ and services instituted by God, and wants his own doctrine and his own services to be observed as divine; secondly, because he takes to himself the power not only of binding and loosing in this life, but also the jurisdiction over souls after this life; thirdly, because the Pope does not want to be judged by the Church or by any one, and puts his own authority ahead of the decision of Councils and the entire Church. But to be unwilling to be judged by the Church or by any one is to make oneself God. Lastly, these errors so horrible, and this impiety, he defends with the greatest cruelty, and puts to death those dissenting.
41] This being the case, all Christians ought to beware of becoming partakers of the godless doctrine, blasphemies, and unjust cruelty of the Pope. On this account they ought to desert and execrate the Pope with his adherents as the kingdom of Antichrist; just as Christ has commanded, Matt. 7:15: Beware of false prophets. And Paul commands that godless teachers should be avoided and execrated as cursed, Gal. 1:8; Titus 3:10. And he says, 2 Cor. 6:14: Be ye not unequally yoked together with unbelievers; for what communion hath light with darkness?
42] To dissent from the agreement of so many nations and to be called schismatics is a grave matter. But divine authority commands all not to be allies and defenders of impiety and unjust cruelty.
On this account our consciences are sufficiently excused; for the errors of the kingdom of the Pope are manifest. And Scripture with its entire voice exclaims that these errors are a teaching of demons and of Antichrist. 43] The idolatry in the profanation of the masses is manifest, which, besides other faults [besides being altogether useless] are shamelessly applied to most shameful gain [and trafficking]. 44] The doctrine of repentance has been utterly corrupted by the Pope and his adherents. For they teach that sins are remitted because of the worth of our works. Then they bid us doubt whether the remission takes place. They nowhere teach that sins are remitted freely for Christ’s sake, and that by this faith we obtain remission of sins. “
Woe to those so-called Lutheran denominations as well that have erred as badly as any pope in their politically correct effort to remake God in their image or substitute a social justice idol. There’s a lot of Antichrist to go around.
Just for reference, your excerpts were from the Treatise on the Power and Primacy of the Pope, which is one of the Lutheran Symbols collected in the Book of Concord of 1580. It was written by Philipp Melanchthon at the Smalcald League meeting in 1537.