Earlier quoted context omitted.
The M-series are ARM chips based on the A series from the iPhones. Therefore Apple had ten years of chip design (plus the prior decades of ARM history) to rely on. The GPUs and modems were a much more recent effort. (My take. happy to be corrected by someone with chip design experience who can comment.)
I would argue their architecture choices long term proved to be prescient (RISC, ARM, etc) and securing TSMC production capacity (beating competitors to the punch) aided them as well.
Their main architectural choice was not about "RISC" or "ARM" but it was choosing "Brainiac" over "Speed Demon", i.e. setting the goal to execute a great number of instructions per clock cycle at a moderate clock frequency, instead of executing a moderate number of instructions per clock cycle at a high clock frequency (the latter variant results in lower fabrication costs, which is why other companies were reluctant to pursue the same choice as Apple).
A high IPC is much easier to achieve when the instructions have a fixed-length encoding, so this RISC principle followed from their main choice.