This policy about which OS your laptop runs does not seem to preclude the use of Windows VMs (and presumably CI runners, though I bet we could see if that was the case ourselves) if it were needed. The open source nature of Gitlab also makes it easy for a developer to test things on an unprivileged personal device, I'd imagine.

If that was not enough, I believe the CI runner is written in the Go programming language, which has a surprisingly decent abstraction of OS functionality in it's standard library. Imperfect as it may be, I suspect that helps reduce the amount of effort needed to ensure individual platforms work properly.