That "100% your own resources" part is key, Microsoft provides a lot of free software. Employees working on their own projects have a "clean" setup that has never touched anything provided by Microsoft. Desktop/laptop, accessories, mobile phones, and so forth.
Heck Microsoft has on occasion promoted apps and tools made by its employees on their own time.
That other companies aren't like this confuses me.
There are rules against working on products that directly complete with existing Microsoft products, for obvious reasons, but aside from that, there aren't any real constraints.
(I've had plenty of friends at MS working on side projects that pulled in anywhere from hundreds to thousands a month, those projects tended to be written against MS technology stacks and made the MS Ecosystem better by their existence, why in the world wouldn't Microsoft want that to happen?)