The poll system call is very likely the same exact implementation for all 4 though. If it was a bug in platform specific code then you'd have a point. It's not like they are shipping 4 complete rewrites.
I don't have macOS 10.12 installed, so I couldn't test it. But I put their test code into an iOS app, and the bug does occur on iOS 10.0.2, on an iPhone 7.
Agreed MacOS/OSX, iOS, WatchOS, and TVOS all are user-land modifications of Darwin/XNU (Next-OS kernel), which is a modification of Mach (BSD-like research kernel).