People keep calling this "a phone chip" when M1 was literally "a phone chip" to begin with, based on A14. It's entirely reasonable that in 4 generations of "phone chips", the A18 reaches the speed that M1 did during its "one-generation" rework.
You are correct that the architecture was indeed the same, but it was quite a different chip. "Literally a phone chip to begin with, based on", to me, misleads from how different the m1 was. But yes, they appear to share the same architecture.