I feel like the scenario itself isn't really "cheating" or NTR in the strictest sense, but it seems like it was very much intentionally written with those concepts in mind. The childhood friend pair is pleasant and huggy and they kiss sometimes - the very picture of an innocent, childish love. I think the reason people are getting NTR vibes is not just because Kanako "cheats" on Ayaka with a senpai, but because the sexual, slightly kinky relationship is explicitly framed in opposition to the platonic one, and that Kanako's sexual awakening is portrayed as a corrupting or degrading influence that results in her losing interest in her ostensibly pure relationship with Ayaka. Moreover, Ayaka's reaction of feeling betrayed yet unbearably aroused and guilty about it is also a fairly typical NTR progression.
I think the story itself could have been told as a love triangle complicated by a disparity of feelings and sexual awareness, but I think the NTR vibes were a deliberate and intentional choice on the part of the author. I don't think it's supposed to be an emotionally complicated story about childhood friends failing to get together; I think it's supposed to be about an intensely sexual relationship proving so much more satisfying than a pure and innocent one, and the audience is almost supposed to revel in the corrupt eroticism of the situation.
I didn't like it, to be honest. It felt cheap, emotionally, and I was uncomfortable with the framing. Given what I think the author was trying to do, though, I guess it was reasonably successful as a work, and I can't really criticise it in that respect. Sure made me feel like shit, though.