Fair question. Im not the author so can only make a guess (other readers of HN surely know far more than I about these subjects). However I suspect the rationale is related to whats been called "Bernstein chaining".
Hmmm, now I don’t get how Bernstein chaining relates to file descriptor interface. I’m curious about specifically the `4<infile` you seem to be admiring, you just observed its unusualness or do you see how its more powerful than the more common `-input infile`?