I think saying "its own parallel gaming stack" probably gives them too much credit [1]. Yes - they put in significant effort, but their gaming stack is neither "parallel" to the rest of the world, nor "their own" in any real sense. They seem to have adapted the open source efforts of Codeweavers, Valve, Wine and the broader Linux community put in Proton and achieving Windows games compatibility on Linux with Vulkan. Adapting that for Metal is no small feat [2] but an investment a giant like Apple can easily make without much risks. Don't forget that Wine has always been developed for Linux and macOS (and the BSDs) in parallel too - it was right there for the taking.
[1]: I'm assuming this is a quote sourced from someone who just isn't aware of all the effort being made at the Linux side of thing in the last five years.
[2]: At a technical level Metal and Vulkan are actually similar enough, but there's just a lot of surface to cover and edge cases to get right.