That's not the same as asserting that soundness requires adherence to the current fashion, sum types over results and errors. TIMTOWTDI.

You don't have to think C is sound or good to distinguish a disfavored API style from mathematical unsoundness. There are infinitely many type safe APIs and languages that are, despite their soundness, ugly and hard to use.

Can you link some?

This is not about soundness. Soundness means that the program behaves in accordance with what its types express. What you're saying is that papers on types show how you can use types to express more constraints.

There can be other valid perspectives and definitions of that word than yours.

I think almost all (tech) arguments stem from simple disagreements and/or different assumptions made on the definition of a (subjective) term.

You may be right in general, but soundness comes from formal logic, where they're pretty fastidious with their definitions. Soundness certainly doesn't mean the expressivity of the types (i.e. what constraints they can represent); it's about the relationship between the runtime behaviour of a program and the types in the code.

Again I think you're trying to push a specific definition as more correct than OP's somehow. Why not just ask them what they meant to understand their intent/context instead of saying they are wrong?

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.