The M chips are mostly just roided out A chips: higher clocks, better cooling, more P cores, big GPU, and I think deeper pipelines and more cache. The ALU and many other sections are, I think, identical.
Thermal throttling is actually a non trivial limit on phones. Put a heat sink and a fan on an A chip and sustained compute is faster.
The OS and its restrictiveness determines the class of device not the hardware.
Like Apple is saying, "Nice iPhone 17 Pro w/ A19 w/ vapor cooling chip you have there; you know you run a full general purpose OS on it, but we're not gonna let you, nanananana :p"
But even as an investor, I think Apple could bring a lot of people/money to the Mac ecosystem by getting them in with an iPhone lapdock.