Only solemn definitions regarding faith and morals are infallible -- an ecumenical council is by no means infallible in and of itself, simply by merit of being an ecumenical council.
Wyrd bið ful aræd is correct that being ecumenical does not mean a council is infallible, though to be infallible it must be an ecumenical one and confirmed by the pope.
However, what is wrong, as shown here , is the belief that non-infallible ecumenical conciliar and public papal teachings do not require religious assent, as non-definitive, ordinary teaching requires "ordinary assent," that being "religious submission of will and intellect," submission of mind and will," which "forbids public contradiction of the teaching," as described here .
Which unlike "sacred assent," "theological faith," “divine and Catholic faith” which infallible teaching requires, allows for internal disagreement, but not public dissent.
And that social teaching which is based on RC faith and morals requires ,religious assent, and that the popes last encyclical is presented as being such, has also been shown (see links), despite insistence that it does not even have anything to do with Catholic faith at all.