Ehhhh idk, afaik jury's still out on that, but imo "could be."

That's partly why I use Delta Chat instead actually. Only partly, but it makes the list.

Even still, gun to my head, I think I'm picking Signal over Matrix as far as who I trust a little more. Not that I trust either entirely, and DC I trust more than both of those, but there's always the possibility of some vulnerability if not a full backdoor. At least DC is FOSS and decentralized, I honestly doubt there's a "backdoor," but never know who has a 0-day.

The CIA financed Signal (I believe they stopped a few years ago), however that was never really secret. It also happened in simpler times, in the earlier days of internet and mobile phones. In general it's quite plausible that the CIA just wanted a really secure comms system that people they wanted to undermine regimes could use.

Contrast this with how the FBI and the Australian feds bankrolled Anomphones, which they then used to read the communications of criminals. That was kept secret until they were ready to brag about it.