I'm pretty sure that they're talking about people deploying hundreds of VMs in a single go... In which case, licensing fees start racking up; and certain kinds of linux are very good at that kind of deployment.

once you start walking away from end-user computing; as an example, MS holds only 8% of all web-facing servers, where Unix (including linux,) is 90%. Linux distros specifically, are something like 61%. Nobody has even mentions apple's macOS server options. Roughly half of professional devs use windows as the daily driver; but that largely has to do with organizations and integration with other MS products (like office.)

IMO, more telling in this conversation is the supercomputers... which are entirely some form of linux. And also, MS's own Azure, clients mostly use linux. And for good reason kubernettes and docker were designed for linux, and it's just easier. Also, there's no per-instance license fees for your VM's if you're using linux. there is for microslop.