These things have categories and tags for a reason.
This doesn't mean much of anything, because words are malleable and subject to change. You may not like it more than I do, but it has happened, does happen, and will happen again. And arguing on a lexical technicality doesn't move the needle, because if we're going to be technical, NTR, is merely a contraction of "netorare" which applies to all lovers of a cheating partner.
Infidelity is not a catch all, especially not for one where the current pair are already cheating.
Except hypocrisy is not the measure. Is it hypocritical to consider the mom fucking a horse "cheating" while fucking her son "isn't"? Yes. I don't think anyone questions that she's cheating on her husband with her son. That's not the issue. It's that she has had sex with someone/something other than her son (her husband notwithstanding.) It's not, "oh she's cheating," and therefore it's automatically "NTR." It's that her sex with the horse constitutes a faithlessness to her relationship with her son, the main male character with whom the audience predominantly identifies.
Cheating is its own tag
NTR is its own tag (and even that has its own subcategories like netorase)
I'm aware.
Someone having more than one partner doesn’t make it NTR, if that were the case it’d have to go next to every harem collection story ever.
You're missing the point, entirely. The female perspective doesn't matter in this case. The reason Harem stories aren't considered NTR because again, the audience is overwhelmingly male. Harem works usually include the participation of all female characters, whereas NTR merely relegates the main male character to either an unsuspecting dimwit, or a forlorn voyeur. You're attempting to create some gender-parity in works that are created to appeal particularly to a male gaze. Ask yourself: why were the two characters, mother and son, as opposed to "boyfriend and girlfriend"? I would imagine that it wouldn't have been particularly difficult to age up the male character. Because Jackerman wanted to ingratiate his works with the incest crowd. The mother and son don't have to be boyfriend and girlfriend; they don't have to be husband and wife; tautologically, they're parent and child. That is their relationship; that is their bond. The incest genre merely sexualizes this bond. And to have a character, human or not, have coitus with the mother after an already established relationship with the son, the character with whom, and I'm assuming this, the majority of the audience engaged this work (otherwise why approach the story using the incest theme?) constitutes a betrayal of the son, and by extension the audience. That is the basis for the transmuted NTR description.
You can argue, "technically this..." and "technically that..." I mean, if Cole fucked a woman who he thought was his mother, but turned out to be a male in drag disguised as his mother, we can argue to the break of dawn whether or not this makes Cole gay on a technicality. And I doubt there would've been fewer members of Jackerman's subscriber base who would've been less displeased by it.