It is really a stretch to say that Leibniz's "best of all possible worlds" argument means he believed in the existence of a multiverse. The centerpiece of the argument is that, of all the potential worlds, this is the one that has to be the best, because it's the one that God created. If the actual set of possible worlds were all created, then there is no reason to believe that the one we exist in is special in any way, which defeats the purpose of the argument in the first place. So the multiverse exists only in the metaphysical sense of "we can imagine a universe where gravity is repulsive instead of attractive," not in any physical sense of "the universe where gravity is repulsive exists next door."

A second point to bring up... Leibniz's theodicy hasn't really gone over well with anyone, particularly any notable theologians. The core argument really boils down to "evil exists because God thought that a universe where it exists was the best one," which is more of a dodge than an answer. Most theodicies instead present evil as somehow necessary to appreciate good or a necessary component of human agency and free will.

As someone else said, Nautilus loves their multiverse theory. Even if they have to twist the words to make it seem like other people believe it.