Yes that's exactly what's happening. Hy has two levels of things mostly: a "compiler" that does the low-level mapping of base python constructs (and whatever can be mapped to AST)... here's where list-comp is implemented: https://github.com/hylang/hy/blob/master/hy/compiler.py#L137...

Anything above that is written as Hy macros, in Hy itself: https://github.com/hylang/hy/blob/master/hy/core/macros.hy

There's a lot of other core code that's written in Hy itself too. https://github.com/hylang/hy/tree/master/hy/core