I just want to know why it's labeled Hentai as a genre when there is a borderline-H tag for its category.

It's published in a hentai magazine, which I believe is the criteria usually used to decide.

"It is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science."

This falls into what was talked about here

Quote from lynira in the thread linked above

Some quick clarifications:
On MangaUpdates, the definition of the hentai genre is strict: "Adult sexual content in an illustrated form where the FOCUS of the manga is placed on sexually graphic acts." So if the intercourse isn't the focus of the manga, it's not considered hentai here. If it's not sexually graphic, it's also not considered hentai here. Please keep this in mind when explaining whether or not a series is either hentai or borderline H.

Because of this strict definition, whether or not a series is hentai on this website is not determined by magazine, it is determined by individual series content (that is to say, it's not considered a demographic here).

As to if it's a seinen, I have no clue, never read it.

Yes yes, I know I make longwinded comments, but that's just me >.<
I should proof read my comments more, but I won't...
So keep in mind I'm filled with typos

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