Earlier quoted context omitted.
They didn't even hit that limit before they sent in the goons in this case, though.
The actual limit is 4x the price of the ticket, capped at $1350, so it could very well have been down in the $400 range.
> For example, an involuntarily bumped traveler who arrives to his final destination more than two hours late (fours internationally) is entitled to an amount worth 400% of his one-way fare (capped at a maximum of $1,350).
There are no regulations forbidding United to offer more when a passenger is already seated, and that passenger thus can't be legally bumped (apart from because of safety reasons maybe).