Does that algorithm detects arbitrary subgraphs with a cyclic component, or just regular cycles? (Not that it would matter in practice, I don't think many people write convoluted mutually recursive mess because it would be a maintenance nightmare, just curious on the algorithmic side of things).

I don’t totally understand the question (what’s a regular cycle?), but the only sort of cycle that matters is a strongly connected component (SCC) in the dependency graph, and these are what get hashed as a single unit. Each distinct element within the component gets a subindex identifier. It does the thing you would want :)