Iirc, the multiple-sources requirement came in because of some cases where there was exactly one "reliable" article on a subject, but it was hilariously bad, like an article on a fringe physics theory in some popular-press magazine that made the mistake of taking it seriously, or an article on a person that paints them in a very negative light. So the worry is that without more that one source, Wikipedia just amplifies bias by parroting what one source says, because the one source gets cited, but the rebuttals don't exist in a citeable form. That can happen with more sources too, but at least there's more chance of sources disagreeing when they're >1.
Despite it being phrased as "notability", to me it's the "verifiability" requirement that's a better argument: there should only be Wikipedia articles on things where there exist enough good sources to write a reasonably well sourced article. That's more of a pragmatic than a philosophical decision; not "this subject deserves/doesn't-deserve an article", but rather "we have/don't-have enough sources to write a good article about this subject".
Despite it being phrased as "notability", to me it's the "verifiability" requirement that's a better argument: there should only be Wikipedia articles on things where there exist enough good sources to write a reasonably well sourced article. That's more of a pragmatic than a philosophical decision; not "this subject deserves/doesn't-deserve an article", but rather "we have/don't-have enough sources to write a good article about this subject".