Why is it even possible to represent a negative PID, let alone treat the integer -1 as a PID meaning "all effective processes"? This seems like a mistake (if not a rookie mistake) in the Linux kernel API itself.
-1 is a special case, a way to represent a PID with all bits set in a platform-independent way. It's not very clean, and it comes from ancient times when writing some extra code and storing an extra few bytes was way more expensive.
Pretty much all the rough edges being discussed here are design mistakes in Linux or Unix, and/or a consequence of using an unsafe language with limited abstractions and a weak type system. But because of ubiquity, this is everyone’s problem now.
It feels a bit like a "better is better" language hitting all of the quirks of a "worse is better" environment.