Self custody is the only way to FORCE exchanges to be honest, right?

I'm not trying to tell anyone what they should or shouldn't do. Too many people are not capable of self custody for numerous reasons.

But am I wrong in thinking that the only way to force exchanges to prove reserves and keep them honest is to self custody?

Otherwise it's just moving 1s and 0s around on their servers where they can say anything they want. It's clearly not legal to do that, but when has that ever stopped banking cartels?