> Do they also need to support MacOS, Chrome OS, and FreeBSD?

They basically do. ChromeOS runs a Linux kernel, so the userland has everything it needs to run CUDA. FreeBSD is a first-class target for Nvidia drivers, and macOS used to be supported too but stopped signing Nvidia code. Nevertheless, AArch64 is still supported for CUDA et. al..

It is up to Nvidia to support whatever platforms they want users to be on. In their case, it means supporting many OSes.