I think they are keeping an evil eye on asahi linux. As soon as that becomes viable, they'll act to keep people on OSX. Their worst nightmare would be steam actively moving to support proton on mac and doing things outside the apple store. Asahi linux would be the pre-cursor to that happening.
If you think that (even with a fully parity with say Ubuntu on Intel Asahi) people would move to Linux on their Mac on any number enough to even raise an eyebrow (much less worry) Apple, you are delluding yourself.
People buy Macs to run macOS. A very tiny minority buys Macs to run Linux on. A slightly larger (but still insignificant) minority will convert an old Mac laying around to Linux.
It's enough to get a project like Asahi support, not enough to make even a 1% dent on Apple's software/services ecosystem bottom line (the hardware side is the same, as those people are still buying a Mac anyway).
Linux on the Desktop - Apple Silicon edition.
They clearly see the Mac a bit differently than they do the other iDevices.
That's a scenario Apple would want to avoid. Hence the announcement of a porting kit.