While this looks cool it is far from useful. I took time to build such cluster with the Pies and shortly after it landed in the box of things I may need in the future. It's just too slow to do anything useful. Watching stuff compile or even boot was like watching paint dry. It is much better to get a bunch of used NUC if you are space conscious or just get VMWare Workstation or similar tool to simulate number of mach…
The person probably enjoyed doing it, most likely didn’t harm anyone in the process, and possibly learnt some things on the road.
Not everything need to be useful beyond the joy of doing it.