Having been doing electronics design since the 1970s I am impressed by the survival of the data.
IMHO, either the memory chips are in a separate sealed module (which can itself withstand great pressure) or the cylindrical capsule did not leak.
If the chips themselves got wet, under the 4km pressure, for 2 years, there would be nothing left because the data is stored in the form of microscopic charged capacitors, and the only protection is the ~ 1mm thick plastic package.
If however the PCB was encapsulated in a suitable epoxy, and this was done under a vacuum to avoid any air-filled voids (which would collapse under the 4km pressure and destroy the module) then the PCB could have survived the total immersion.
It would be interesting to know how these things are made.
It's awfully hard to make a watertight package which can hold 4km pressure for 2 years and which uses just o-rings...