Yeah, I watched that followup interview with Lex Friedman and the way he says it, it sounds to me like they have backdoors into the baseband and IME that they use for targeted surveillance. They don't need to hack the cryptography, they can just get all data going into memory on your device.

That said, I am skeptical or Signal due to the phone number requirement and mandatory central server. I'll use it, but only with people I know IRL and only for casually sensitive information, if I had anything going on in my life for which I needed real deal security, I wouldn't use signal and in fact I wouldn't even use a cell phone for it at all.