Kiiind of.
The memories he KNOWS he's missing are the ones from most of his time in the west sea rift. We know that already.
The memories he doesn't know he's missing are the ones we see here in the memorial dungeon.
But explaining more is spoiler so.... in case you don't mind more spoilers
SPOILER >>>>
> there are no "alternate universes". J created this memorial dungeon in the first timeline. The present story doesn't take place in the first timeline, though.

And i want spoilers on whose memories are the memorial dungeon based on?