The kid in the video is just learning and in an exaggerated position at the bottom, long torso and short legs- you can stay very upright and get deep, short torso and long legs- youre going to have to create some forward lean to get to the same depth. Trying to stay perfectly straight up and down and go deep for someone with a short torso/long leg body type is going to be murder on the knees.
You didnt look too far forward on your latest squats at all, it looked alot better than the previous high bar ones you posted, that squat is going to suit your body type much better and allow you to go deeper without killing your knees.
Well incidentally, I have lower back pain now, and I've had it two weeks ago as well, from the squats. And I feel like I'm bending forward a lot. Flip is doing the same as well, he's lucky he has the shoulder or whatever (pec) flexibility to get his palms over the bar and hold it as he's supposed to but other than that... he kind of bends a lot at the waist when doing the low bar squats.