I think you're confusing the parent's use of "primary" + "source" with the lexical item "primary source". The parent presumably meant "one of the most main sour

I think you're confusing the parent's use of "primary" + "source" with the lexical item "primary source". The parent presumably meant "one of the most main sources [to use as one's only or first point of research]". Sort of separately, but as threeseed points out, Wikipedia is frequently used as a corpus for primary research, so technically it's that kind of "primary source" too.