Might want to read Update from the CEO: Part 1 - #59 by gez-mycroft to start with.
Pi zero 2 was released more recently than would have allowed for its inclusion; the mk2 started a few years ago, and the kickstarter was premised with the pi3 at the time. The pi zero was around, but wasn’t sufficient to smoothly run mycroft. The zero 2 fixes the CPU, but still limits the RAM, so ends up being a compromise as well. The radxa zero defaults to 512mb, so be sure to get the 1gb version if you do want to try it. The cpu on the radxa is the s905y2, which by all major benchmarking tests is closer to a pi 3 than a pi4. It’s arm’s A53 architecture, the pi4’s bcm2711 is A72.
On top of that, the sj201 board is almost the size of a pi itself–it handles the audio and buttons. The lower part of the mk2 enclosure is speaker box (and thus empty) to help sound quality, and even the upper part has a great deal of empty space.
There’s a number of ways to fix pi heat issues (sinks, fans, etc) that could also be tried, the mk2 has a fan, I haven’t seen how it’s positioned on the production models. Add a heat sink and it’d be hard to overheat it. The production version of the mk2 supposedly reorients the fan and pi board positioning to better handle the heat flow.