Seems the author made it public domain not because that'd be the best for the software industry at large, but because "holding on to it any longer benefits nobody", which means the amount they earn from it monetarily is today minuscule compared to what it was.

Parents point seems to be about that if this author of Slug wouldn't have patent it in the first place, then in general that work would have had an larger impact in the software industry, compared to this situation where the author did patent it.

Case(s) in point: all the comments on HN who independently came up with the same algorithm but couldn't publish it as FOSS because they found a patent covering it.