If final ping represented fuel exhaustion in the cruise, we need to explain why the plane apparently ditched on the same arc as fuel exhaustion. This also requires plane to cover perhaps 120km in last 8 minutes of powered flight (= 900kmh or approx 480 knots). Even if plane was flying perpendicular to arcs, speed would still have been 380 knots (50NM in 8 minutes).
However if ditching would you really wait until fuel exhausted? Surely you would prefer to have a bit of control over the last few seconds of flight.
If had power at time of ditching and the final ping represented the ditching itself, what could account for final partial ping? Possibly loss of antenna. This scenario requires plane to cover same distance as above, as well as losing altitude and slowing down for ditching in 8 minutes.
On balance, this suggests ditched after fuel exhausted. So perhaps it flew along the arc when ditching. Or perhaps was already very low altitude in order to assess sea conditions when fuel exhausted, and ditching only a few km further on.