I mean, is the girl even dating yet?
Unless she's started dating already with the supposed mc of the manga (in the manga) then isn't she still free to be taken?
It's not NTR if she's still single.
NTR would still apply to the situation.
The most common depiction of NTR is with someone in a romantic relationship but the jealousy/stealing away aspect still applies when one character is infatuated (reciprocated or not) with the one being stolen away and hasn't yet built up the confidence to confess yet.
So in a meta way the manga-in-the-manga (where the wimpy guy is the protag) would still be netorare but this shift in perspective (the MC becoming the stealer with wimpy guy as the supporting cast) makes
this manga netori, even if inadvertently with MC trying to not have it happen.
The NTR genre has weird twists and turns like the differences of girls love v. yuri or questioning whether or not lolibaba and oppai loli are true loli.