Seinen? It says Shoujo on the main entry page, and Josei on wikipedia.
Lol, whoever put Shoujo there was high.
So actually there's no demographic here. Seinen, shoijo, josei, shounen, those are dependent on the magazine the work is being published in. These are target demographics, not genres, despite what people generally assume.
But indeed, because most of the industry, especially when it comes to Shounen, is cookie-cutter slop, words like that very "Shounen" in practice act like genres. Shounen gives a feeling of cheap power fantasy catering to young male teenagers that lacks any kind of serious, thought-out world-building or story. Seinen is like "better Shounen" in that regard.
More real, better written, yada-yada.
But those are generalisations.
Anyway, this work is a Pixiv/Twitter online comic, so there's no demographic. But if using these words like genres, it's certainly not a fucking shoujo (just lmao, shoujo is stuff like
Aishiteruze Baby or anything published by the famous
Hana to Yume (花とゆめ) magazine. Just one look at the covers and you
know it's Shoujo. You
know when you see shoujo, one of its key stereotypical aspects is males being the pretty ones and girls being the plain ones. It also has a very specific artstyle often that is drastically different from the more widespread male-targeted manga.
In my opinion,
Senpai ga Urusai Kouhai no Hanashi is pure Seinen due to it being an adult cast in an office environment (both things relating much more to adults than to teens) about classic male fetishes like Jock-Loli and Onee-san-shota, and having
males plain and
females the stars.
It might be unintuitive, but male otaku works feature plain males hooking it up with hot girls because this way more
ugly NEETs average Joes would be able to self-insert.
In female-oriented works, the reverse is happening: two bishounen princes fight over a plain, unattractive girl.
As a rule of thumb, if a very crude one, you can deduce this way: "Who's the cool and who's the hot one? If men are cool and girls are hot, then it's the male demographic; if reverse, then it's the female one.
Then, "Are they adults or teens?" If they're kids, then it's for kids (duh); if adults, then it's for the adults.
Of course, exceptions abound. For instance, in Romance works, the love interests are often cool as heck.
Incidentally, the Wikipedia article to
SenUru states it's Jousei, but first of all it was published by a generic publisher, so what gives, but also see my message above. Joisei is stuff like
Sakamichi no Aporon or NANA (even though it's technically Shoujo, but Shoujo the
genre does
not talk about serious adult women talking serious real adult love stuff).
TL;DR: It's either "no demographic", or it's your average Seinen in a "it's so simplistic it's Shounen, but they're working adults so I guess it's Seinen" way.
Cheers/