There are a lot of theoretical articles which claim similar things but on the other hand we have a lot of empirical evidence that ARM CPUs are significantly more power efficient.

I used laptops with both Intel and AMD CPUs, and I read/watch a lot of reviews in thin and light laptop space. Although AMD became more power efficient compared to Intel in the last few years, AMD alternative is only marginally more efficient (like 5-10%). And AMD is using TSMC fabs.

On the other hand Qualcomm's recent Snapdragon X series CPUs are significantly more efficient then both Intel and AMD in most tests while providing the same performance or sometimes even better performance.

Some people mention the efficiency gains on Intel Lunar Lake as evidence that x86 is just as efficient, but Lunar Lake was still slightly behind in battery life and performance, while using a newer TSMC process node compared to Snapdragon X series.

So, even though I see theoretical articles like this, the empirical evidence says otherwise. Qualcomm will release their second generation Snapdragon X series CPUs this month. My guess is that the performance/efficiency gap with Intel and AMD will get even bigger.

I think both can be true.

A client CPU spends most of its life idling. Thus, the key to good battery life in client computing is, generally, idle power consumption. That means low core power draw at idle, but it also means shutting off peripherals that aren't in use, turning off clock sources for said peripherals, etc.

ARM was built for low-power embedded applications from the start, and thus low-power idle states are integrated into the architecture quite elegantly. x86, on the other hand, has the SMM, which was an afterthought.

AFAICT case for x86 ~ ARM perf equivalence is based on the argument that instruction decode, while empirically less efficient on x86, is such a small portion of a modern, high-performance pipeline that it doesn't matter. This reasoning checks out IMO. But, this effect would only be visible while the CPU is under load.