Why the multiverse is religion, not science

Because some random person on YouTube with half a knowledge of epistemology decided it is?

First, the video says "Scientists say that something exists if it is useful to describe observations".

Sorry, that's epistemology, and one particular epistemology, not science. Scientists work at another level than that (which is a meta level).

Second, falsifiability and "usefulness to describe observations" is not the be-all end-all of what is science. That's again a particular epistemology, and while a useful general guide, it has been found wanting and not historically matching what real scientists do and how actual scientific theories are formed and established.

Third, there's nothing that says whether the multiverse can or cannot be tested at some point (in fact there have been some theoritical and experimental attempts).

Fourth, religion also has other aspects (moral code, ritual, spiritual element, holy scripture, etc) not just "believing in something that's not useful to describe observations". So even if the multiverse merely matched religion in one aspect, doesn't make it a religion, as it doesn't match those other aspects.

Finally, from the view point of religious people themselves, religion is considered very useful to describe observations -- e.g. this or that happened because of God, or this phenomenon is a manifestation of this or that's God's wrath, etc. So even if that criterion (whether something is useful to describe observations) was enough to distinguish science from religion, it wouldn't help us do so except if we were already with the side of science (and so it's a circular argument).