If this is true, find a way to FIRE THOSE PEOPLE OR FORCE THEM TO USE WINDOWS!
I don't care how great they are (on paper), or how kewl MacOS is, they should either walk the walk or get out. I'm sure there still are more than enough designers willing to work on products used by 1 billion people.
Imagine the opposite scenario for Apple and how absurd it sounds: designers at Apple designing features for MacOS or iOS using Windows OS laptops.
This prompts the following question: What design apps are the Microsoft designers using on a Mac that they can't use on a Windows machine? (Is it Mac-only Sketch?)
The main designer tools, as far as I am aware, are bundled in the Adobe suite and that works on windows, as well as Mac.
But there are tons of other designer apps, that are Mac only - but I would assume it is mainly a designer thing to have Macs - they have been succesfully marketed (and to some excent were designed) for creative folks.