You can't always buy patents. If other players don't want Apple in the CPU market, they could just block Apple. Patents offer a monopoly on a technology after all. A different scenario would be if Apple built a war chest of CPU-related patents, which they could use for trading.

Apple isn’t using any instructions from their CPUs

There's all sorts of CPU related patents that aren't related to an individual ISA.

Then how has Apple not gotten sued over the Ax series of processors? Or the iPhone, the iPad, the PPC Macs, etc?

Because they have their own warchest of patents from acquiring about every decent processor startup over the past 15 years that would allow them to counter sue, and microarchitectual details are under incredibly strict NDAs to where it's an uphill battle to even prove that Apple is using any of those patented techniques to begin with.

Also, the PowerPC macs weren't their chips, those were IBM and Motorola for the most part (Apple did have some input into Altivec, but didn't do anything from the RTL down AFAIK).

You are contradicting yourself. Apple hasn’t been sued because of their patent portfolio, but they still have that. Little changes with this transition, they are still building computing devices around the ARM instruction set.

I'm not contradicting myself; all I've said is that patents matter across ISA boundaries, but Apple has a lot of patents.

If another company were to try this without a warchest the size of Apple's, they'd be sued into oblivion.