It's always been 10x pricing for macOS on GitHub Actions which is absurd from the start BUT it's even worse since the machine they give you is horribly slow. Like it will take 2x longer to check out your repo and install dependencies than it will on linux which really means it costs 20x more. I've considered doing a 2-step flow where I build part of my app on linux, put the artifacts somewhere, then start up macOS and build the rest of the way (iOS app using Capacitor). But at the end of the day the cost is still way lower than buying a build server mac so I just put up with it and pay it when I go over my included minutes.
They mention that they use Apple Virtualization Framework, that means they use real macs with macos but the github action runs virtualized on it through AVF