likely need to add the docs to this too...

I know this is evil.

That's not true, they get passed in different registers in some ABIs (amd64 uses XMM registers, aarch64 uses D registers). Though like the in-kernel printf floating-point formats aren't even supported in the implementation so whatever register it uses doesn't actually matter, it'll just stop processing formats and print the rest of the string verbatim. So it doesn't hit UB, but it also doesn't work as intended.

How come? The userspace versions work just fine...

This seems odd, why condition it?

This sounds like something that should be fixed in libunwind with something like _LIBUNWIND_HAS_NO_FLOAT or inferred from soft float compiler flags. But I guess it's okay as an ugly workaround.