They didn't say the language was the issue. It doesn't support the full C spec. But if you want a reason why language might be an issue for a compiler, it could make compilation time slower. But I think the point of this project is not real world use but fun demonstration of skill
Maybe not the language choice, but the codegen of this compiler is terrible because of the single-pass shortcuts (for example, it unconditionally loads the result of all assignment operations back to the stack just in case you want to write `a = b = 1`, even though 99% of the time that load is immediately thrown away.)