They're talking about the volume 10 bonus page for that chapter:
https://mangadex.org/chapter/3c593f87-4932-42ea-8316-6c96d8f51d1b/12
The "deluge of rejections" was "gomennasai rush" in Japanese, referring to the flashback in the chapter with all the girls rejecting him by apologizing. Otherwise he's unambiguously calling it his fault.
For what it's worth from what I remember there's nothing saying he had to be rejected 100 times specifically, it just works thematically. Regardless, I personally find arguing about these kinds of specifics in a
gag comedy romcom series that clearly didn't put all that much thought into its opening premise (especially since it was essentially abandoned by chapter 3) kind of unfruitful.
So that's where the other guy got their argument from.
But well, my point still hold that you still can interpret that as 100 rejections being the side effect of having 100 soulmates simultaneously and the Love God simply admitting that perhaps this will not happen if Rentarou had only 1 soulmate like others normally.
As for debating for canons of a fictional story, it honestly depends on how consistent it is to its own logic. IMO despite being a gagrom, 100 Girlfriends is actually pretty logical and consistent when it comes to its own canonicity, thus i think it's worth to argue about specifics to make sure any misunderstanding can be ironed out early before it become a common misconception among casual readers.
For example, you said the manga drops its own premise after chapter 3, i take it you're talking about the death penalty? Well, it's actually not. The very chapter the bonus page you brought up is for literally explained why the "death penalty" thing is put at the backburner, and occasionally you can see glimpses at how death will come for some of the girlfriends if they didn't work out well with Rentarou:
-Hakari almost fall to her death,
-Kusuri almost got kidnapped by the evil organization that targets her,
-Tama is fine with being starved to death if Rentarou didn't "pick her up",
-Kiki almost got run over by Athlete Club the moment she "rejects" Rentarou.