@Solistia I think you meant to write shoujo or shounen. But that's what's written: https://ncode.syosetu.com/n5577es/
Pretty sure it also qualifies as a light novel if we take the informal definition. So I don't think you were wrong to think it was a LN. As said previously - what a publisher labels a product is not necessarily always correct with regards to the usual definitions. This is made quite clear when you consider that other publishers may tag and have tagged works differently after licensing it. Thus only objective qualities such as readership statistics and writing techniques/tropes can give a somewhat objective answer to the question regarding the target audience.
@Rugid
Target demographics being decided by publishers, period.
I disagree, and I already explained in details why. It also doesn't help that what you wrote doesn't agree with the common stance regarding this - go on, google it.
that's the only official definition.
This is clearly not the case, do your research before making statements.
Because publisher decided it. Simple as that and there's no need for wild guesses based on personal opinions or some "tropes".
You realize that to decide it, the publishers have to resort to what you are discounting in your second sentence, right? Also after licensing a work publishers have been shown to change tags, thus it should be clear this is a subjective categorization.
Do you know what's not subjective though? Statistics. Number of adult/teenage male to female readership ratios, but unfortunately you usually can't get your hands on those. That's what correlates with whether a magazine is labeled seinen/shounen/shoujo/josei. Doesn't mean you can't have typically shoujo works (as in works that will fit in a shoujo magazine just right) in a seinen/shounen magazine - it just means that the majority of the works are tailored to target audience X.
So the next least subjective option is analysis of the text. The thing you seem to denounce for some strange reason.
Granted, I believe that if you were to take a poll, you'd get a very teenage female skewed ratio regarding this manga/LN, simply because of the obvious elements in the text. That's as far as my opinion goes, the rest of the stuff I wrote you can verify in a somewhat objective manner.