the thing is, people are attributing it to ARM, rather than how Apple handles their OS. its the sole reason why Snapdragon X Elite wasn't that great on Windows, because ultimately, the problem wasn't about x86 vs Arm, but it was about how windows handled low powered operations. If valve makes a piece of hardware that's arm based, they clearly aren't going to be using OSX for any reason. You can tell by the discussion because you can easily name which generation processor you run on a MBP, but fail to mention the cpu models for either the AMD nor intel powered machines and gives the aura of equivalent playing fields when it fundamentally wont.

Just because Apple with their heavily controlled OS space can make the transition to ARM work flawlessly for batterylife doesn't mean it applies to all other ARM devices. Arm definitely does some aspects better, but it's not by default better in every situation due to the nature of the environment that surrounds said hardware is. The power efficiency only exists if all applications are recompiled to target said hardware. For a gaming device, it's not going to be very useful because very few games that Valve would target have an arm based build. You get into the problem that emulators have. things like proton is a translation layer and suffers much less overhead (e.g why mobile phones can do switch emulation for instance(arm to arm based translation layer) but no phone remotely will do ps3 emulation (arm to ibm cell processor), despite console wise, being roughly the same in performance.

It's the sole reason why Apples dev kit for games doesn't run games like proton does(where it can legit run games better than original if its using an older API). Because architecture changes isn't just a translation layer, theres a layer of emulation to it, which while can be hardware accelerated if done right, is never 1:1 like a translation layer is.

Want to test how your MBP battery life is on a different environment not entirely tailored to Apple, run Asahi Linux for example and you will notice immediately that the battery life isn't the same. (asahi linux is a fedora based distro tailored for M series machines)