Rather than boring everyone with more of my bullshit I'll just offer a plausible explanation for people who hold this viewpoint.
You could always just reason that Toriel gave Eve the sweater, Hard Mode is probably non-canon if canon is even a thing here. The description mentions that the monsters are "Crazy" in this version. So maybe Toriel is a little nuts and is coping by trying to pretend Eve is a child or something for reasons, making her wear the sweater and everything. Toriel being like "Stay in character!" lol I'm probably putting more thought into this than I should but fuck it.
I still think it was meant to be a joke line that people took seriously though.