I feel like the story could still work even if the guy didn't get isekai'd to a 1:5 world. The author could just establish that the men at the girl's school and town don't match with the kind of guy she's looking for, and say that the male mc just moved in or transferred to the school. With this premise, this chapter's story would still happen the same way.
What new or unique thing does the 1:5 isekai do for the story then? As far as I can tell, it gives a reason for the girls to be greedy and the guys to be arrogant. But this can still happen in a normal earth-like world. So the 1:5 isekai world doesn't really give a compelling narrative hook because there's nothing definitively unique about it, in my opinion. Like, an isekai where every human had 6 fingers on their hands would be a unique premise, but if the whole story was just a high-school/college romcom then the whole story could have happened on normal 5-fingered earth and nothing would change.
At least for the 1:39 story, the author leans super into it by building up all these systems that revolves around the giant ratio disparity. They are extreme scenarios, but they come from an extreme premise.