If we're comparing entire systems as products you're absolutely right. That's not what this discussion is about. We're trying to compare the efficiency of the ISA. What do you think would happen if AMD replaced the x86-64 decoders with ARM64 ones, and changed nothing about how the CPU idles, how high it clocks or how fast it boosts?

My guess is ARM64 is a few percent more efficient, something AMD has claimed in the past. They're now saying it would be identical, which is probably not far from the truth.

The simple fact of the matter is that the ISA is only a small part of how long your battery lasts. If you're gaming or rendering or compiling it's going to matter a lot, and Apple battery life is pretty comparable in these scenarios. If you're reading, writing, browsing or watching then your cores are going to be mostly idle, so the only thing the ISA influences won't even have a current running through it.