It's an intensifier. As a native English speaker you should probably be aware that we eventually sand-blast the semantics off of words until they all becomes synonyms for "good", "bad", "very", or "um". (This is similar to what French does to phonemes, but unrelated.)

The author is just expressing amusement at the surprising simplicity of the resulting algorithm

    Adverb
        hilariously (comparative more hilariously, superlative most hilariously)

        1. In a hilarious manner; so as to amuse greatly.

The author was greatly amused how quick the resulting algorithm works.