And what's the use if you don't know they're not compiling against a backdoored version under the hood?

That's a different question.

No, you're just moving the goalpost (and you're pretty bad at it too)

Perhaps I’m bad at it because I wasn’t doing it at all?

You asked how someone can trust a crypto implementation that isn’t open source. I replied to the to it directly: it actually is open source. Personally I see the source being available largely irrelevant but I replied to exactly what you asked for.

Your second question is an entirely different topic, which is how you can trust that something isn’t backdoored. Notably, this has nothing to do with whether source is available. How I would typically do that is by inspecting the compiled artifacts themselves, which is the same whether the code is available or not. Of course, this requires that the OS or the AP or the crypto engine isn’t backdoored, for which there exist more involved verification processes. Whether this is possible to do in general is a difficult research area. It is, however, completely divorced from your view on how this works because auditing the properties you’re looking for does not rely on source code at least in a traditional sense.