Yes, it's this. I also own an M4 mbp and an AMD framework 13. With both on maximum screen brightness, side by side, doing similar workloads, battery life isn't that much better on the M4. I think the difference maker is that the mac constantly decreases screen brightness when possible, turns the backlight completely off when there isn't any activity, heavily leverages power efficient scheduling and efficiency cores, no doubt turns off power to all peripherals whenever possible, and so on. And of course lid-closed suspend on a mac lasts indefinitely. Arch does none of these things and even on cohesive distros like Fedora there's only so much you can do in user land. Linux is designed for compatibility across a huge breadth of devices; darwin only has to support Mac hardware and can extract every ounce of power efficiency from deep hardware integration.

IIRC the low power states of M series chips generally dips down further than most x86 CPUs do, and the way both the SoC And OS are designed are for racing to idle and coalescing tasks to reduce wakeups. On the MBPs specially the screen can also drop down to 1hz so the GPU isn’t wasting cycles redrawing static content.

The result is that in more typical usage where the machine isn’t under a constant load, battery life is much better. When it’s sitting there idle displaying a web page it’s barely consuming any power at all, where most competing laptops at minimum are pulling at least 2-3x as much power between the CPU not being able to scale down that far and constantly getting woken to perform poorly scheduled tasks.