To actually get your satellite into a new orbit that doesn't intersect the original, you need to maneuver again: in this case, after you have followed the new (elliptical) orbit for ~12 hours (half an orbit) to its new high point.
All this is complicated by the fact that an explosion is further acceleration that shifts the orbit of the debris.
(That said, they appear to have ample time to move the satellite, it's a question of rules that would ordinarily prohibit it. The race is with the bureaucracy. Also, since the thrusters are designed for stationkeeping instead of propulsion, it's more of a gentle spiral outward than two fast maneuvers. Finally, some of the complication is about having ground tracking stations that can communicate with it: they have to speed it up by going lower, causing "eastward drift", before they can slow it down by going higher, which will give it westward velocity on the surface.)
Source: I used to work for NASA.
The reason a single short burn won't do it is that whenever you stop burning you will always be in an orbit that will return you to the point where you stopped (assuming you don't actually collide with the body you're orbiting of course). So you have to burn somewhere other than the orbit you want to get out of. The most efficient place to do that second burn is at apoapsis because that's where you get the greatest reduction in kinetic energy for a given delta-V (because that's where your velocity is the highest and so the force of burn is applied over a greater distance). But if all you have is a low-thrust engine (e.g. an ion drive) you can maneuver with long burns. The math gets hairier though.
> They have excess propellant on board they can't dump fast enough which is the energy source of the explosion (the battery failure is just the trigger)
Normally they don't let satellites in the graveyard with any propellant as an explosion there can add unpredictable amounts of delta-v to the satellite and one or more husks that it connects with post explosion :-).
Personally I'd suggest they just start boosting out of geosync and keep going out until they run out of fuel but not enough time for that either it seems.