ES is just having fun with QM. The thing that interest me is whether or not a multiverse exists in which all the possibilities you can find after a measurement are realized. Even if the many worlds interpretation is false and QM refers to a single universe, you can still have a many worlds like scenario in an infinite universe where an infinite number of copies of observer exist (e.g. in eternal inflation models).

Also, one can imagine an infinite ensemble of universes. It could be that there is no distinction between physical and mathematical existence. We could be purely mathematical entities living in an abstract mathematical world, experiencing our own world subjectively as something real. All possible mathematical models would then define their own universe (such a model has been proposed by Tegmark).