I know of developers who made quite a bit of $$$ being first out the door with applications utilizing SDKs that they wrote. MS is perfectly happy with their own developers showing off shiny new features.
Interesting but not surprising I guess.
On the day of employee orientation, they specifically talk about moonlighting and HOW to do it.
"Do not use any hardware we provide for you, and do not use any software you installed from the corporate network, buy your own copy of development tools and licenses, you can use the free Azure credits we give you but not the ones linked to your employee login, make sure you use the credits on your personal account."
They do require you not write software that directly competes with existing Microsoft software, for obvious reasons, but other than that it is fair game.
But this doesn't even cover the whole of it. At multiple times, Microsoft has actively encouraged their employees to write software as a side hustle! They've launched internal competitions "We have a cool new SDK, write some software and put it on the MS Store! Best rated software wins some prize!"
The entire "developers first" culture there extends to how they treat their own employees.
Heck MS has a long tradition of employees leaving to start a new company and MS buying that company up a few years later. I suspect that during the 90s/early 2000s it was seen as a good way to innovate, similar to incubator labs within companies now days.