>>58154260
The games and the anime both say those locations are their home dimensions and is the reason they are the ONLY pokemon in those world.

they are an invasive species pokemon that completely takes over their environments. Look at Ultra Plant and Ultra Crater. The entire geoscape is covered with only Celesteela and Xurkitree. These extremely powerful fast populating pokemon take over their planets.

>>58154410
>Celebi has you defeat Giovanni in the past
>This somehow doesn't effect the timeline.
I'm just saying, Celebi seems to be able to manipulate time without causing any issues.