The wish is a level 9 spell, one of the uses listed there is:
The basic use of this spell is to reproduce any other level 8 or lower spell. You don't need to meet the requirements of this spell, including the expensive components. The spell simply takes effect.
The question is at what level is this copy of the spell cast? I can see three possibilities here:
- The wish was cast from a level 9 spell slot, so the copied spell is cast at level 9.
- Wish can be cast up to a level 8 spell as a copy, so the copy cast is cast as if it were using a level 8 slot.
- The spell being copied is at level N, so the copy is launched as if it were using a slot at level N.
There are good reasons to cast low-level spells at higher levels, the additional die per level of healing to heal wounds, the additional missile per level for magic missiles, etc. a low level would generally be undesirable, but the fate of wish is unclear.
The first two possibilities make sense and are useful, in the example of the 9th level, you simply get the spell as if you had cast it normally in the slot, in the example of the 8th level, you sacrifice a casting bonus level for the ability to cast possibly unknown, unprepared or cross-class spells (no restrictions against copying cleric spells like a wizard with wish) – and this utility seems to be a fair compromise.
But which one is "officially" correct?