I mean, taken to the extreme, I can probably read through the source of VLC and know that it's correct, given enough years to read it and study video compression standards etc. Does that mean I don't get use out of someone else having written VLC for me?

Knowing something is right and producing it are completely different things. You might be thinking of ChatGPT too narrowly, as a simple question-answer thing, but even now you can ask it to write code that will save you time, and scale it up by a few factors and it's doing the equivalent of writing new libraries for you. (Probably many years before it can write VLC for you, if ever.)