I'm confused. 1) Are you saying these decrees issued by the Church are or are not infallible? 2) Is Vatican II infallible? 3) If Neither of these documents represents the infallible teaching of the Church, how do you know they are correct?
What decrees? The list above are errors, things taught or spoken that are erroneous or wrong.
2) Is Vatican II infallible?
Vatican II was an ecumenical council during which no dogma or "doctrine" was promulgated. Nothing new is taught as a result. Someone with more Church knowledge than me can answer whether or not an ecumenical council is infallible. The reality is that much of what was claimed to be changed by Vat II was, in fact, not. That there were no concluding anathemas has been a bit of a thorn as nothing clear-cut came as a result other than misrepresentation to further agendas.
3) If Neither of these documents represents the infallible teaching of the Church, how do you know they are correct?
The syllabus does not change what was always taught. It just states it in a different way. Vatican II didn't change Church teaching, no matter what the progressives say, if you actually read the documents, so there is no reason to doubt the veracity.