I don't think it replaces the Pokémon typing and its only considered towards the offensive.
For example. For a Pokémon like DDNite with Roost + 2 attacks, you can either reinforce your STABs (like, this is a horrible nite set, but im using it as an example) via crystalization, or augment one of your non-stab moves (like EQ).
This is interesting as a competitive option, since you have to choose between reinforcing your already solid dual stab combination to even higher levels of damage (im thinking about setup sweepers with 2 offensive moves that are unresisted by a majority of the meta and a third that doesnt need the crystalization boost), or you can keep your stab moves at x1.5 and gain a third stab on something, but i doubt this crystalization type will be counted on the defensive side.