@DM_Ronin @GrapheneOS let me preface this by stressing that I use signal, and don't really care for telegram as a messenger. All I'm doing here is being curious.
Regarding the things you linked: Green's link is just a lot of stuff about signal, all he says about TG is that he doesn't know how MTProto works or "thinks" it's insecure in some way.
The second link is just the default problem and says nothing about encryption weaknesses(victim probably didn't use secret chats).
The YT link does an audit/validation of the Mtproto model, discloses some vulnerabilities that they say have been fixed.

Wiki states that the model of MTProto 2.0 was audited by Italian researchers. This doesn't really say anything about weaknesses in the current implementation.