I got the impression that they weren't particularly looking for a hero pool with a "deathball" meta, and the hero pool they selected had more to do with those being some of the simplest heroes to program at first. There is a lot of overlap between with the heroes they implemented and the set of introductory heroes that are recommended for new players who are playing their first games.

Regarding the insane reaction, I wonder if there is a natural way to handicap the AI to more human-like reaction times. Reaction time is not a good measure on its own because human reaction time can vary a lot depending on the level of surprise.

That said, my impression from watching the game was that the power of the AI had less to do with perfect reaction time and more to do with their "hive mind" coordination. If an enemy ever gets in the wrong position, they are immediately punished by a concerted attack. Humans have a harder time doing this because they need to communicate their intentions first. Sometimes each player will be focusing their attention on a different target.