It's open source, so you can check the code.
Funny thing, there are lots of contributors to matrix stack (protocol, sever, etc) and only some part of them are New Vector employees, because it's open source. Again, you can check that yourself in matrix repos
Sure, but open source doesn't mean it's free from bugs, unintended or intended, and if any intelligence is behind it (again, I don't know enough about this, so I'm not saying it is), then you can bet that there are some intended bugs in there that will allow said intelligence to at least listen in.
I agree about bugs, but doubt any intelligence can use fully open source tools to integrate backdoor or at least "ears".
Of course, I'm not employee of any intelligence and can't prove that, but that's just my opinion
> What the heck is insecure about that? Other than vague promises that the Jews have my data?
I mean, if it's connected to any intelligence (I'm not saying it is, nor do I know anything about Matrix's history), it being secure may just be a facade.