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.