Dumping is only usefull if you really need to squeeze the last performance out of your plane. You dump 2.5 tons a minute and speeds go down 1 knot per 5 tons.
So the fact that they were dumping suggests a performance issue, ie 1 or more engines out.
On the other hand there are strong reports of heavy smoke in the cockpit.
Unless these are related (ie airco smoke due to contaminated bleed air from a partly failed engine.. quite remote in this severity I might add), it is a multiple failure. Not unlike a sim-scenario after all.