Not at all, they're not even in the list of the contributors, as you can see on the 4.16 kernel statistics : https://lwn.net/Articles/750054/

The list doesn't vary too much with the versions, so either they're working under the radar as "Unknown", or they barely contribute to the kernel.

Microsoft contributes a bunch of money to The Linux Foundation now but they really don't contribute very much code. Their main concern seems to be keeping Hyper-V working well so that Linux VMs are performant on Windows server. They don't register as a top code contributor in the last Linux Kernel Development Report: https://storage.pardot.com/6342/188781/Publication_LinuxKern...