@Amarrez NovelUpdates needs an update if the WN is indeed not Josei (then again, the update should probably entail it being changed to shoujo). Also arguing that the base material has no bearing on the adaptation is strange.
There are multiple reasons why this reads as a shoujo manga. The sexualization of the main character is there only to further the Mary Sue trope of everybody admiring how amazing she is - and yes that includes comparing how amazing her figure and sex appeal is compared to her "rival" (if you can even call that fodder character a "rival"). Needless to say, if this was targeted at boys, it would have been the other way around.
The fact that there's no love interest introduced (yet), or that the emphasis is not on that doesn't really entail anything - not all shoujo manga/novels have an emphasis on this. Granted it may run in a magazine where mostly "seinen" manga are published - I do not discount that, however, I believe that it is clear that the major elements in the manga are there to appeal to a teenage female demographic. I can go in even more details if necessary, and do a page by page analysis and identify the tropes and techniques used to appeal to said female audience (those are well known tropes too, so that is a non-issue), and contrast those to ones used when targeting a teenage male demographic.
Taking into account your answer to Rugid, I think I have a better idea where you're coming from. And I wholeheartedly agree that:
Manga doesn't have to have explicit sex scenes or gore to be a seinen, that's a ridiculous notion.
On the other hand, a manga requires somewhat realistic character interactions if it is to have a pretense at being targeted at adult men or women. Clearly a plot that is pretty much a Mary Sue celebration where realistic interactions are nonexistent doesn't exactly fit in that category.
As mentioned the sexualization (if I get correctly which part you're talking about) is there only to emphasize that our MC is superior in every possible aspect (except possibly regarding empathy and morals, but being harmless is not a virtue either).