I still don't understand what did Eliezer mean by "knowable".

It's one thing to say "there are multiple competing interpretations," another to say "there is a simplest interpretation, and other, more burdensome variants," and still another to be able to point to the calculations that determine which explanation is simplest. I don't poll people on their interpretations of QM, but I am of the impression that many people who have opinions on QM interpretations aren't even aware that there's math that is useful for comparing interpretations. (Hence them not realizing that it's "knowable.")

There's also the point about lightness, and using the most likely explanation as the default explanation, instead of saying "sure, it's more likely, but not so much more likely enough that my pet theory is totally unacceptable in comparison, so I'm sticking with my pet theory."