Also, the experiment method has some flaws. Problems are hand-picked out of a random subset of the full set. Why not run the full set?
Most likely cherry-picking. The approach is only going to work well in domains where Prolog is commonly used to write solutions to problems, like logical puzzles or constraint problems etc.
Since you seem like an expert: is there a better technology for logical/constraint programming? I loved predicate calculus in school so it seems like there should be something out there for me, but so far no dice. This seems kinda related to the widely-discussed paradigm of “Linear Programming”, but I’ve also failed to find much of interest there behind all the talk of “Management Theory” and detailed mathematical efficiency comparisons.
I guess Curry (from above) might be the go-to these days?
More generally there are the theorem provers like Coq, etc., but their use cases are even more specific.