This is a semantic problem, not a physical. Generally, there is no single definition of the word 'universe' that all physicists ascribe to. When you have causally disjoint set of spacetimes that may or may not be governed by the same set of physical laws, some people choose call these multiple universes.

Similar to what happened with the word atom, it literally means indivisible, but instead of renaming atoms, we just kept adding other names when we found out the atom was actually very much divisible.

The assignment and communication of meaning is a fascinating topic/problem that affects pretty much everything we do, yet it's usually invisible or transparent to us most of the time.

It is more about studying the universe within a framework than about going through a magic door to other universes. For example, if you take a mathematical group one thing you can do is view every group as a group of permutations. That changes the "framework" but it is equivalent to working with an alternate encoding of the group.

The universe of discourse on the whole still considers this space to be a matter for discussion.