The contract states they can throttle when AT&T "believes" he is impacting their network. So whatever criteria they use, if it is their belief, then they're right. They should probably have put a sole discretion for any reason clause, but still, it is crystal clear.
This guy just wanted money, and figured he could sue them, and did it. His claim is not valid, and that will be proven to be the case in the appeal.
In his interview one key point that helped him win the case was that he was being throttled at 1.5 Gbytes which is way low. I understand if he was being throttled at 5 Gbytes but 1.5 is outrageous. They also throttled him differently every month.
He will need to prove he was throttled at those times. AT&T, just like every other service, does not guarantee service or speeds at any time. He has the burden of proving that there were limits on his speed specifically enforced to him at any given time. Although AT&T admitted to throttling his speeds when he tethered or went to a certain area - it hasn't been proven or dis proven it happened at a specific certain amount.