I'd guess Linux is the most important platform for most customers. Also, it's probably the easiest and least expensive platform to support. I'd be very surprised if they'd picked another platform to support in their initial release of this feature.
The "every job uses a Docker container" is what worries me. Because the other two certainly don't have a robust container mechanism. Sure, Windows has something, but it is a far cry from "pick a distro, install some stuff, and call it a day" because there's some blessed image that supports installing .NET stuff and if you don't base on that, you're SoL (at least last I checked). And that's even ignoring the lack of rendering if you need it or the filesystem performance hit they seemed to be plagued with.
There's the exec runner which can be used without any separation, so it's not very secure but doesn't require docker...