I’ll preface this with agreeing that you’re probably correct.

That said, it wouldn’t surprise me at all if Meta built an intentional backdoor, and that someone else (or many someone else’s) found it and was utilizing it.

If such a backdoor exists, it is probably cryptographically secured to prevent "unauthorized" access. E.g. the xz backdoor was secured like that.

Or that the government offered Meta $50 for a list of agitators and they said why not. Given Meta's track record it's totally on brand.