Hi, guy who helped Ruben set that up here. There’s a few bytes of memory in the .sav file that are persisted outside the individual save slots and can be used to set cross-save metadata. On a brand-new .sav, those are all zeros. We write a nonzero value there in the epilogue event, and then the new game+ stuff checks for that value.
This guide here really isn’t very helpful, but is where I first learned about it: NewGame+ (FE8)
