So why is it a non-goal of Roc to be implemented with this approach?

Have you not read the post? Because compile times. Rust has awful compile times. If it didn't, I'm sure the Roc team would have stayed with Rust.

Yes, I read the post.

I don't understand why the goal is not to (eventually) implement Roc in Roc, maybe with a dash of something else for the "platform".

Roc is pitched as a general purpose functional programming language with great performance.

How does a compiler not fall under this category?

Roc is pitched as having great performance for a GC'd language, that is, on par with Java, Go, C# instead of Ruby, Python, JS. The Roc compiler team are looking for C, C++, Rust, Zig kind of performance. Roc will, by design, never reach that kind of speed.

Go's compiler is written in Go, and is known to compile very quickly. I'm not sure I understand why Roc needs to be even faster than that?

We already have many great GC'd functional languages at that performance level.

I first learned about Roc from this talk:

https://www.youtube.com/watch?v=vzfy4EKwG_Y

That's a niche that is not currently filled (well, maybe MLTon) and that has me very excited! I'm sure I'm not alone here.