I mean, using this definition, you could be 'self-informed' about ANY topic - as long as you have the skills, time, and desire to fully immerse yourself in the subject.

And, as you point out, it is literally impossible to do this for every subject - even if we all had infinite aptitude to study and understand all subjects, we don't have the infinite time it would take to do that. Plus, this is an extreme waste of our resources; we specialize for a reason.

We need trust, and you can only spot check that trust every now and then for things that you actually know.