Have you read up on general/ecumenical councils and their related infallibility? I suggest you start there.
Bottom line: if this was a true, valid ecumenical council, then it is to be considered infallible. If a so-called “ecumenical council” taught error, then it can not be a valid council of the Catholic Church. Those promulgating such error can not be legitimate authority.
Comparing Vatican II with the Arian heresy is a false comparison. Individual bishops believed in the Arian heresy. They did not, as a whole with the pope, teach the rest of the Church the Arian heresy as Church doctrine in an ecumenical council.
So if a "true, valid ecumenical council" teaches that all the faithful must wear green shirts on earth day and sing hymns to Gaia, such a teaching must be considered infallible?