As meandog pointed out, the writings of these men are not to be considered Scripture.
These men were practicing Catholics, attempting to reform a corrupt Church.
They were, at the least, just as Catholic as the SSPX.
Are the SSPX Catholic?
I don't know a lot about Zwingli but Jean Cauvin aka Calvin was the son of a secretary to a French bishop or archbishop and his education was fully paid by the diocese or archdiocese in question. While Cauvin was brought up Catholic, he apostasized promptly after graduation and removed himself to Switzerland to avoid the consequences. His Scriptural differences with Rome were more over matters like free will vs. predestination than about corruption. Yes???
SSPX attempts to keep doctrine, dogma and tradition (save the last forty years) intact, while making disobedience to legitimate authority a way of life. Luther, Zwingli and Calvin shared te disobedience but did deviate rather substantially from doctrine, dogma and tradition or these threads would be generating conbsiderably less in the way of activity and posts.did not.