Remarkable that no one yet here, including the article author, reports the true origin of these section numbers: they identified (depending on section size, one or a group of) physical binders in the series published by AT&T to document System V UNIX, and when you got an update to your system software, it came with a package of new manual pages which you would physically install in the binders to replace the now-superseded older versions. Everything you hate about man pages is in consequence of that origin, and of the corollary that the online version was never designed to be authoritative.

I have one of those physical binders, a volume of Section 3 for an AT&T 3B2, in the software section of my library downstairs. A beautiful artifact in every respect, of the level of quality you would imagine in the manual for a machine that cost $15,000 in the 80s.