Yes a force applied to an unstable part of the body that has more leverage, i.e. a knee or a head, can cause a loss of stability resulting in a fall. That's not the same as "knocking someone down" IMO.
The guy who just hit the ground would feel differently about that.
The trooper in the video got hit in the chest and the bullet was stopped by the vest yet he still went down.
As I mentioned there are a number of similar videos with very similar results.
If hit by a bullet and they fall over because they've been hit, then by definition they have been knocked down.
If a lesser caliber bullet won't achieve the same effect then that lesser bullet has less knockdown power.
You may say that if you are well protected and aware that you are about to be shot , and the strike is to the center of the chest with thick ballistic plates absorbing the shock then you can more than likely keep your balance. But saying a person can not be knocked down by a bullet impact, under quite a few real world conditions when they are taken completely by suprise, would not be strictly true.
My older brother was a golden gloves boxer. he once asked me to punch him in the stomach as hard as I could. He off course was ready and braced. I hit him with no effect.
I then said lets try that with your shirt off. When he was pulling his t-shirt off over his head I unloaded on him and knocked him back several steps. He was bent double and white as a sheet.