There are many equally valid "true" endings, not counting all the side endings. Sounds like it wasn't your cup of tea, that's fine. No VN is universally loved.
Also bonus link to one of my favorite tracks in the game: https://youtu.be/dQ5g1Va7CjI
Most VNs are structured as routes. There are a handful of independent stories. The reader not choose how the story will unfold, but rather chooses one of several linear stories to follow. I would classify that as a fundamentally linear story. The fact that the route is chosen by a choice in the middle is not really different from choosing the route from a menu at the start.
SubaHibi is even more linear than that, because the playing order of the routes is enforced. There is one "True" ending, which is required to be completed last and is the only one which resolves all the mysteries.
(I say the True ending resolves all the mysteries, but the mysteries were resolved to my satisfaction by the end of It's my Own Invention. That wasn't by design, though.)
Anyway we aren't going to agree here. I am interested in two things: what VNs you do like... and what VNs you would consider non-linear. The discussion can't really continue without a point of comparison.
(And, in my opinion, it's one of the worst things ever written.)