There was a recent episode of Upgrade where Jason and Myke interviewed a couple Apple execs, and they talked about processor scalability being a major focus. So you start with a base (the A14 currently), then scale it up or down depending on what you're putting it in. A prime example of this is the A12X/Z processors in the iPad Pros, which added more cores to provide more performance over the A12 that the design of the iPad allows for while not requiring the software to target something new.
The Mac will likely take a similar route, with more cores and higher clock speeds to take advantage of the cooling capabilities of a Mac, but all built as an extension of the A14. Then, as the A15 is developed, all the SW has to do is target one new processor, and that'll work across all the devices that run it, from the Watch to the Mac. A special new "Apple Silicon" line would really throw a wrench in this, and complicate a clean and simple development strategy that can move all of Apple's products forward together. So I think "Apple Silicon" is really just that, a general term for all silicon designed by Apple.