You're mostly right about the history of Machinekit AFAIK. The reason they broke off was because LinuxCNC took a long time to merge changes as they focused stability. Machinekit guys wanted to push the envelope and focus on small devices and multi-node setups. The big difference was MK had it's hal stack split from the "CNC" stack. They separated out the part of LinuxCNC that had to run close to hardware and real-time from stuff that could be done by a remote computer over a network. They did this because they were heavily focused on things like the beaglebone black, which has a tiny ARM CPU and like 1gb of RAM but it also had a special realtime processor which made it good for controlling hardware. The idea was that a small version of Linux and just the HAL side of LinuxCNC ran on the BBB, while some other computer somewhere else with better graphics can run the GUI and everything else. MK also had support for newer GUI toolkits...for a while anyway
MK also did alot of really neat things with obscure hardware. MK splitting from LCNC was probably the worst thing either could have done. LCNC has the support and user base, MK had alot of the really talented people. MK failed pretty hard on keeping up with support for Mesa hardware and fell way behind LCNC when it came to stuff like that. They got so far behind that they dropped most of their own hal stack and they port LCNC straight into MK which is why the github has separate hal and CNC repos now. I was working on something nutty a few years back and spent alot of time with machinekit just to realize that support was just falling off a cliff and I had to drop what I was doing as it wouldn't work with LinuxCNC, Looks like the git repo is still somewhat active but the Google groups is dead. the fact that their main discussion happened on GG rather than a forum is probably what killed it.
I would personally stay away from MK, without the key guys supporting the thing it's dead. LinuxCNC has moved on from the archaic GTK2 GUI's and now supports QT better than MK did. BBB's were never really a thing in LCNC but Rpi4's are. Oddly enough I don't think MK ever ran on an Rpi. Mesa makes a couple of cards specific to Rpi4's so if you're looking for a small footprint that might work well.