My point was more that there's no reason to question a tag whose reason for existence is plainly evident and correct upon reading the work, and that I felt that this particular manga is a reasonably straightforward case. (i.e. it's true that there's plenty of ambiguous cases in other works—but the relationship in question here isn't a subplot or something between side characters; rather it's the entire point of the plot, and the center of most of the jokes too).
Also, manga updates does indeed have this listed as shoujo-ai, as of the present time. (It's possible that someone corrected it only after seeing your comment, though.)