No, it isn't. That what it has become, because of people that don't get what it actually is. NTR is specifically about someone stealing someone elses lover. Keyword here is lover. If the characters are not in an established relationship, it's not NTR. Full stop. There are other tags now for what you describe, where someone's love interest is taken before they get a chance to confess, or someone's mom/sister is getting fucked or whatever, but that is different.
Simplifying the term to the point that you have results in innocent devs like the one making this game getting bombarded with accusations by people who have no idea what they are talking about. It also makes it much harder for people that actually want that content to find it. It also makes it harder to avoid that content, because if tags are misleading then people ignore them. The scene had no NTR connotations at all, unless you are obscenely insecure. At best, it had NTS or sharing implications, and even that feels like a stretch at present, since the MC is not currently in a relationship with anybody in the game. He just met them.
People crying NTR all of the time are just as annoying as people begging to have NTR added all of the time.