I did not write anything about "singular they". I wrote against the legitimacy of those linked guidelines, which accidentally include "singular they".

The poster I replied to stated (I remember) that some use of language is made legitimate by some group laying out some guidelines. That some pretty random group («not philologists») lays out guidelines is maybe an "affiliation pass" for their group, but not universally valid.

You need grounds, good grounds. Check those guidelines...