I don't see anything that says that this is a Microsoft product. It looks like a side project by a Microsoft employee. (full disclosure: I work at MS.)
> Not open source
Looks like the source code is available at https://github.com/BabylonJS/Babylon.js under the "Microsoft Public License" which has been certified by the Open Source Initiative as an open source license.
On the flip side, there's no reason to hate everything Microsoft do, c# and Mono are great. Maybe people should get over dismissing anything open source that Microsoft do, but the rest of what you've said makes no sense.
Microsoft didn't do Mono, it did .NET which Mono cloned in a form supported on a wider variety of platforms.
There's a lot of reason to not use MS technologies even if you do like MS.
That was 20 years ago!
According to the Free Software Foundation, it is a free software license but not compatible with the GNU GPL.
That doesn't mean much, the GPL (v3) is incompatible with itself (v2). Not a GPL hater, just pointing out that it's not a big deal for licenses to be incompatible with the GPL.