"If you have to debate if it's NTR or not", facts decide yea or nay not your feelings.
The entire concept of NTR is based on
experiencing feelings of anguish and/or betrayal because someone the protagonist has feelings for is engaged in sexual acts with someone else. Facts actually mean very little for people who get invested/incensed in this genre, and for many of them it counts as NTR if they see a character they (the player) wants as their RO has sex with anyone else.
These people really don't care if your definition is different, and will let their displeasure known, loudly and repeatedly.
As for this particular game, apparently it was the dev kink to repeatedly trigger that sort of people with possibility of NTR, through cliffhangers, etc. He's been doing it for months and, like all kinks, repeatedly doing the same eventually loses its luster and ones feels the need to escalate. I'd guess this is simply the escalation to "but it's not technically NTR" so "more snowflakes complain" and the author gets his fix.
