une forte accélération verticale
It seems rather sloppy text even in french. Surely it should say velocity or speed, since I don't see how it can be deduced whether or not the aircraft was accelerating at the time of impact.
Acceleration in this context is physics / engineering speak for the forces on impact.
In layman's terms: It hit the water going pretty much straight down in normal flight attitude.
That does not preclude horizontal or rotational components on impact, but the deformation pattern led BEA to say:
Les observations effectuées sur la dérive et sur les éléments de la cabine passagers (office, porte de toilettes, module de repos PNC) font apparaître que l’avion a vraisemblablement heurté la surface de l’eau en ligne de vol, une forte accélération verticale
If significant distortions in other planes were apparent, I expect BEA would have noted that in the preliminary.