I think I understand their point and even said, "What you're saying is that papers on types show how you can use types to express more constraints," and that's correct. I merely pointed out that that's not soundness, which is something I think they'd like to know. There may be some leeway in the definition of soundness and how it's similar to or different from type safety, but either way, type expressivity is something different altogether (and C certainly has issues with both expressivity and type safety or soundness, but they're different issues). So the point isn't to win - the argument was right and I'm not claiming that C's type system is expressive - but to explain the different terms and their meaning in programming languages and formal semantics.