They probably beat AMD/Intel on perf/power efficiency, which is why it makes sense for MBA and 13" MBP.
The smaller machines are also likely held back by cooling solutions, so if you have Intel beat on power efficiency in a tiny form factor, you can boost your clock speed too.
Considering how AMD also beats intel on power/perf by a wide margin and they compared their results against Intel CPUs I wouldn't be surprised if their power/performance was close to AMD (they do have heterogeneous CPU cores which of course is not the case in traditional x86)
Apple's perf/watt is much higher than AMD. Anandtech has the A14 within a few percent of the 5950X on single threaded workloads. Power consumption is 5 watts (entire iPhone 12) versus 49 watts (5950X package power).
Normalizing for performance (say A14 vs 5950X at 4.5GHz instead of 5GHz) would close the gap somewhat, but it's still huge. Perhaps 4x instead of 10x - those last 500MHz on the AMD chip cost a ton of power.
Of course, none of this is particularly surprising considering Apple is using both a newer process and gets nearly 60% higher IPC.
AMD's Smart Memory Access would like to have a word. I'd note that in unoptimized games, they're projecting a 5% performance boost between their stock overclock and SMA (rumors put the overclock at only around 1%).
I would say on your topic though that Intel has a hybrid chip that might interest you. Lakefield pairs 4 modified Tremont cores with 1 Sunny Cove core.
I think that we will see how this pans out in practice - but power efficiency is not something I associate with Intel - they just seem to far behind in manufacturing process to be competitive.
We don't even have new Zen 3 mobile chips yet if they are anything like the New Desktop parts we just got last week then everything Apple just put out is FUBAR.
I'm hoping Zen 3 delivers because 4000 mobile chips shows great promise but is not available in anything premium (MacBook pro level) and has supply issues.
That being said Apple has an advantage in that they control the entire stack and on the power efficiency side of things they have experience with mobile CPU design, they can do stuff like heterogeneous cores and can optimise for it throughout the stack, I think it would be challenging for x86 to do the same.
The smaller machines are also likely held back by cooling solutions, so if you have Intel beat on power efficiency in a tiny form factor, you can boost your clock speed too.