In what way can statements about a multiverse not be ontological? Are you committing to modal realism, Platonic realism, or are you just pulling my leg?
If you commit to the multiverse being physical, by extension you're making claims about ontological concerns, since all physical claims require there to be a notion of something existing, even if you're wanting to claim that something counterfactually exists. Then that gets into modal realism etc etc.
Likewise you can't be fully confident in multiversal theories since no physicist endorses a multiversal theory without also accepting it as interpretative, as no experiment has been performed to anoint multiversal theories as being correct. So you're still postulating something worthy of criticism even if there was nothing ontological involved.