I honestly don't get how this is even a problem. My best friend is a UX/UI designer who works on Mac but the guy also games and used to do crypto mining back in 2013 so he is very familiar with Windows as well. Why is Microsoft having problems finding designers that use and know both?
It's a problem for so many reasons it's not even funny, from the mundane to the esoteric-but-important!
A random example: Macs these days use screens with a brightness response Gamma curve of 1.8 and PCs use 2.2. Macs use a color gamut of DisplayP3 by default now, which has different color primaries than the PC sRGB standard.
Why does this matter? Because two shades that are easily distinguishable when physically viewed on a Mac may not be easily distinguishable on PC! Colors wills shift, and even 50% grey is represented differently. The same JPG or PNG file from a design will look different on a PC even if it has an identical monitor as a Mac!
Similarly, font weight, anti-aliasing, and kerning are very different on the two platforms. Any text design done on one has to be adapted for the other. It's not just the default fonts! The same font at the same size will look very visibly different.
Did I mention default fonts? Only Times New Roman, Arial, and Courier are common between the two platforms. All of those are physically different fonts that look different even if transported to the other platform.
Well, not entirely their fault. Your GP (now my GGP) began by stating:
>> I honestly don't get how this is even a problem.
Which can be interpreted not only as "How can this even happen?", but also quite reasonably as "So what's wrong with that?". The parent comment to yours (GP to this) answered the latter.
Why are they allowed to work on Windows? Even car salesmen are often required to drive the same car as the one they're selling.