Keep in mind that the diversion happens well before the aircraft is "running on fumes." It happens at a point well before the aircraft has the minimum fuel to reach the alternate airport. (I think it's something like when there's only enough fuel to reach the alternate airport plus 30 minutes spare fuel after that.)

Kinda like a company will declare bankruptcy well before they have zeroed out their cash reserves. (Sorry, I've been spending too much time on the AA forum where we keep wishing for bankruptcy...)

It is certainly possible that an aircraft could load exactly the FAA required amount of fuel and still need to divert if there is excessive holding at the destination. My guess is that this is what happened here.