According to him it takes about 5 hours to get to the command prompt which is kinda funny but interesting project nevertheless

I'd imagine that a modern kernel has a lot of pointless fat in it and that the 5-hours figure could be reduced if perhaps only slightly, with appropriate optimization effort. It might even be useful to deliberately have such a super-slow (but reliable, since it runs on highly deterministic microcontroller-class HW) benchmark in the mix, as you would be able to make very precise and fine-grained measurements about how even seemingly-unrelated changes might impact performance within the kernel, whether for the better or worse.

I believe that the idea would not be relevant in "general purpose benchmarking improvements" due to the vast differences in micro architecture between this CPU and modern CPU's.

The pipelining and internal optimizations differ greatly.