Marketing, probably, but also accuracy. Apple integrates a bunch of specialized co-processors as well. They want you to think of this, and the Arm bit, as one unit.

Yes. They want to take as much credit as they can. This isn't a dig at Apple, it's a rational decision.

Because you can't buy a Chromebook with the "superior" Apple Silicon. You can only buy a Chromebooks with an "inferior" ARM chip. I personally like that naming. It gets the idea across. Apple is not part of the ARM ecosystem. Apple is it's own isolated island.