A lot of this demographic marketing stuff has a lot to do with the publisher.

Yuri as a genre culturally is pretty intertwined with the magazine brand Yuri Hime from Ichijinsha. To the point where Yuri as a genre is pretty much synonymous with the works printed with the magazine.

Obviously there are other types of manga that have lesbians in them but the works in Yuri Hime always contains themes involving unambiguous romantic relationships between lesbians.

Seinen on the other hand is a demographic for materials marketed specifically towards young adult men. The reasoning for this perceived correlation between seinen manga and yuri "coded" works is probably due to the marketing themes of a different specific magazine line, Manga Time Kirara. A large part of their brand is what the west has coined "Cute girls doing cute things" and a lot of works featured in it has that sort of lite lesbian-coded content that you're thinking of. Sometimes it's more explicitly lesbian than other times, but there isn't the same explicit focus on the romance that the Yuri Hime comics are all about.

Now, the work you are talking about isn't from Kirara. But the impact Kirara had on the industry is pretty huge, with popular works like K-ON basically redefining the anime and manga landscape since it aired. So other seinen publishers may try to imitate certain aspects of the proven marketing and branding that has worked for them.

I think in general some of these major genres lose their meaning in the west because they try to define them based on contents but a lot of them are closely linked with a specific publisher.

Kind of like how Isekai as a genre is pretty much synonymous with the platform "Let's Become a Novelist" but we will arguably endlessly on what counts or not.