I think they also may be referring to some Bitcoin Core authors recommending against other implementations - the concern is if the implementations contain an er

I think they also may be referring to some Bitcoin Core authors recommending against other implementations - the concern is if the implementations contain an error in the consensus code, they could accidentally fork off, and potentially lose funds for the user. For similar reasons, Bitcoin Core builds reproducible static builds and recommends those, the minimize the chance of behavior divergence. But you're right - it's merely a suggestion from a portion of the developers, and it certainly doesn't stop anyone.