It seems pretty clear to me that this is for Valve's long-awaited standalone VR headset, Deckard. What other reason would they have to support Gorilla Tag on ARM? https://x.com/SadlyItsBradley/status/1837210246076588500 Compatibility with your existing Steam library out of the box will be a huge competitive advantage for Deckard over Meta Quest.
Macs would be a big juicy target, for example. If they can get emulation working well enough with proton on that, that's a huge market. Lots of people with disposable income on there that could be buying lots of games. And Apple is doing similar things as well so there might be some opportunity to tap into what they are doing.