"Apple Silicon" is simply a marketing term for, well, Apple's silicon. They're making this major change to their computers with the promise of better performance, functionality and battery life. Apple's made major investments to have the best chips in their class. Many of the functional improvements come from SoC siblings like GPU, neural cores, hardware codecs, etc. Why would Apple want to give ARM all the credit for that?

Apple isn't shy about communicating to developers that Apple Silicon uses ARM ISA. This is about conveying to consumers their unique platform advantage.