Sure, but Apple isn't selling their silicon to anyone else and Valve, successful as they are, don't have Apples money and economy-of-scale to throw at designing their own state-of-the-art CPU/GPU cores and building them on TSMCs state-of-the-art processes. Valve will have to roll with whatever is available on the open market, and if that happens to suck compared to Apples stuff then tough shit.
This doesn't feel like anything Apple has done in modern times. The last thing I remember them licensing was the iPod+HP from 2004-2005. Apple barely does enterprise support; they're very focused on selling their products to consumers and I don't think they're at all interested in selling CPUs to others.
Apple waffles and sometimes talks about gaming on Macs, but they lack the commitment that is needed. A lot of people like to buy a game and continue playing it for years, even after the developer went on to something else; or to buy years old games on sale. But you can't expect to run a mac os app compiled three to five years ago that is media and gpu heavy intensive on today's mac os. There will have been mandatory developer updates and it won't work.
Win32 is the only stable desktop ABI... and games need a stable ABI.
The Nintendo Switch already provides >160 million reasons for gamedevs to care about native ARM support, but that hasn't moved the needle for the Mac. Being ARM-based is the least of its problems, the problem is that it's a relatively tiny potential market owned by a company which is actively hostile to the needs of game developers.
The switch is underpowered to the point that most A(AA) games cannot run on it without a ton of effort and compromise, an M chip powered device would be a different story. But anyway it's never going to happen, just daydreaming about a perfect gaming setup...
I'm definitely dreaming but I think it could be a win-win situation if Apple decided to licence its chips to Valve: the resulting handheld and VR headsets would be power/efficiency monsters and PC devs would finally have a good reason to target ARM, which could finally bring native PC gaming to MACs.