Is it truly possible to have a solid understanding of the interface and its use, though, without having a reasonable insight into, at least, the likely implementation of something?
I'm sure this isn't what you meant, but how about something like movement? I've met doctors who know anatomy like the (ahem) back of their hand, but they couldn't perform coordinated physical tasks to save their life.
Yes, that's the whole point of abstraction, to reduce cognitive load by ignoring irrelevant implementation details.