At the risk of being "irrelevant", I wanted to nail down exactly what
assertion is being made here: are we talking about directly executing
in assembly code the relevant x86[-64]/ppc/whatever instructions for
"read-and-clear", or going through OS-dependent access routines like
Windows' InterlockedExchange()?

We are talking of the assembly code. See file byterun/signals_machdep.h, which contains the corresponding macros.

Regards,
Markus