As in, the java compiler formally provides extension APIs, and Manifold taps directly into those?

Hm, ok, in that case, slightly less hacky. Probably more comparable to using gcc extensions then... in that, I'd still hesitate to call it "this is evidence that c supports x" ... but it's the kind of statement that could be argued both ways, and discussions would probably quickly devolve into semantics and pedantry rather than anything of practical significance.