Awesome tackle, that's as good as you'll see. Everything about that is legit, perfect execution. iThink is 100% right, as far as form tackles are concerned it's up there with the best of them.
I don't know why they would flag that. If you're putting your head/shoulders in someones stomach/hips area you're naturally going to take their legs out from under them to protect yourself, because if you don't take their legs and "slam" the person you're the one who's going to get sat down on your a.ss.
Bending down to tackle low as Barron did, back straight, is one of the most vulnerable positions you can put yourself in as far as head-on tackling is concerned. To protect yourself from concussion (or simply being owned) it's necessary to 'sweep' the legs and, with the help of momentum, slam the person onto their back.
Surprised they flagged it