There are two versions of the game (two technically different game "builds", as it were): 
  1. The PS4 build, which can also be launched on the PS5, 
  2. The PS5 build which can only be launched on the PS5
If a player started playing DL2 on a PS4 and has bought a PS5, he or she will require to launch the PS4 build on their PS5 to be able to use any old save files.

The player won't be able to use their save files created with the PS4 build when playing the PS5 build. 

See also:

Is there crossplay and can I transfer my saves between gaming platforms?