I always saw it as Galeem using Shadow Kirby for Kirby clones.

I've seen a few arguments about this idea.
1) Since Meta Knight is also whatever Kirby is, Galeem can make Kirby copies using him.
2) Kirby travelled so fast he has entered a new dimension where a completely different Kirby failed.
3) For the same reason you can have three King Dedede's at once in Star Allies. Game logic.