Secondly, while I'm no fan of Meta/Facebook (and think they're a net negative for the world), I trust them way more than Signal to get basic facts right in their statement. Facebook is a massive corporation with lawyers and PR people who know that outright lying about something like this is a horrible and possibly illegal idea.
It seems pretty obvious to me here that Signal expected Facebook not to comment on this and for them to net a juicy PR win. On the other hand, Facebook has no reason to believe Signal would stay quiet after being accused of lying if they had a shred of evidence to the contrary. Unfortunately Signal achieved their goal anyways, because we still have people like choppaface coming out of the woodwork to defend them because "Facebook would have done it anyways".