I only read the report regarding AttackerX so my reply is solely based on that incident and Meth's decision.
I denied it because actually there is not a rule regarding this. Futhermore, he was just watching a video in youtube like he said, that can't make him lag so hard, his lag it's because a slow internet speed, everyone have the internet speed which can afford, if his ping is over 500 he'll get kicked when he spawns, denied.
As Kakarot pointed out, that's basically what artificial lag is. Whether it's third party or not it still affects gameplay.
Furthermore, youtube can actually be a major contributor to lag.
Sfi7z for example starts lagging with 1000 ping with just youtube videos opened in the background and he has as you said not a fast internet connection (just 5 MBs, look at his signature) but still manages to retain 230 ping average ingame without anything interrupting his connection.
With that said, I know for a fact that AttackerX is not playing clean due to the fact that his ping normally is around the 200. not 400-500 or beyond, he usually starts using crap when big events are ongoing like for example the WK gathering from a few days ago.