Ok, what are the back-of-the-envelope computations that he should have done before starting to build this?

Pi memory bandwidth is less than 10 GB/sec, so AI use cases will be extremely limited. Network I/O is maximum of 1 Gbps (or more if you do some unholy thing with M.2 NICs), so that also limits maximum networked performance.

But still can be decent for HPC learning, CI testing, or isolated multi-node smaller-app performance.