While tooling can be better on Windows, and many do target it first, I took issue with the claim that no one targets anything but Windows.

Anecdotally, I just had Atmel recommend a switch from Windows to Linux, as their compiler toolchain "can work better there".

This space is more diverse than just Windows, and growing in that direction more and more. Atmel, Cadence and Qualcomm are just a few who welcome you with open arms.