If the choice is a C library with 95% of the code in inline assembly, ... C is not providing any benefits in that situation, is it?
That's the whole point, it's less about the language, and more about the specifics of the code itself in a testable way. Saying something is written in C without accounting for a bunch of inline assembly is analogous to saying something is written in Rust and not accounting for a bunch of unsafe blocks. Not in that they are equivalently safe or unsafe, but that that high level assumptions because of the language fail because of the practices done within.
If I had to choose between two libraries written in C and one was pure C and one was 50% inlined assembly, and I viewed security and safety as more important for my use case, I know which one I would choose if that information was surfaced to me easily.