just checked my TH9C, mine has a small notch there as well, however mine is a more of a clean oval shape. I don’t think it is anything to be concerned anout. The video was quite negative on the trigger pull, my trigger was gritty at first, Got a little better after seneral hundred rounds but not great. After cleaning I used some dry graphite lube on the internal action components this made a BIG difference, grittyness is gone and single action pull is smooth and fairly light. Also sight alignment from the factory was way off both front and rear sights were off center so you may want to check that on you gun too.
My TH9C will NOT work with Winchester Forged steel ammo. Failure to feed and/or failure to eject. I polished the feed ramp and bore but that did not help. Brass ammo is not much more cost right now so I will stick with brass.
The more I work with it the better I like this pistol, @$280 all in for a new pistol l think it is great value.