No Apple wont be entering that market and Apple GPU's are mobile/productivity focused, they aren't good for games.
Someone can make a Arm system that could run SteamOS but only provided the drivers are open source and built into the Linux Kernel which is what Valve and SteamOS rely on (this means no closed source drivers).
State of Arm GPU's.
* Apple GPU - not supported in linux kernel, a custom linux distro exists for Apple Arm hardware with many drivers that are outside of kernel, will take time to bring those in.
* Arm Mali - Arm's own in house GPU's do have good Linux support but not the latest generation, it takes while for those to come up support wise.
* Qualcomm - Very good support in Linux, latest Snapdragon Elite X2 GPU is in the processing of being added.
* PowerVR - only in the past few years or so have started to properly support open source drivers under Linux.
* AMD - open source driver works fine with Arm.
* Intel - open source driver works fine with Arm.
* Nvidia - viable RTX open source drivers in development but unusable unlikely to be used by SteamOS till it matures.
So no there isn't going to be some kind of rush on Arm Steam Machines any time soon, the only one that is really viable right now ( in next 2 or so years) is Qualcomm because Valve will already have done some of the driver and firmware legal work to make it all work under linux via the Steam Frame.