Not sure why he would have thought this would be possible. This would be an extremely hard problem given the latency involved between different nodes.
Then he came across VCL, or Virtual Open Cluster... “It did just what I wanted, not with an entire OS per se, but with an entire OpenCL application. and that’s good enough for me.”
A similar (but far older) system is MPI[1], which enables parallel computation across many compute nodes for your code by providing message passing. It's kind of a pain to use in my limited experience since you have to adapt your code (it seems like VCL is transparent for OpenCL programs), but it does work. No need for OpenCL, tho of course you could always use OpenCL + MPI. A common thing I see is MPI+OpenMP (for parallel cpu computation).