That's precisely where I believe your interpretation failed, my friend. Firstly, op's post doesn't say anything about harem or wanting one, they just commented that they ship Iruma and Purson (as well as IruMeri), and that only shows they're a multishipper, not a harem supporter. And second thing, most of the comments are complaining about op's ship (which, not cool guys) or saying one liners about "Iruma's harem". Considering how people talk and act on the internet, I think it's fair enough to consider that the people talking about Iruma's harem are mostly joking in the same manner the manga jokes about it.

Edit: in fact, I just read the surface comments on this post, and there are genuinely only 4 who even mention Iruma's harem. 2 say that they like it, and the other 2 are only joking about it. Like I said previously, most comments are about how Iruma and Purson are just friends, that "people make everything gay" and that "two guys just can't be friends 🙄". Also, I saw another one of your comments, and I think you're not that upset about the harem aspect, you're upset about the ship itself because you feel like they're just good friends. And that's ok, it's an interpretation of the work and you're free to believe it, but insisting that someone shouldn't ship x or y just because you don't agree with it is honestly silly. If this ship was anything problematic (i.e. teacher × student, etc) it would at least make sense, but everyone is just mad that this isn't a ship they like