Well, I’m a believer in RQM but this is not a sound argument in the least. It’s so unsound I can hardly understand how it constitutes an argument at all. Firstly, of course one can assign a probability in this situation, not based on an “ensemble” but based on ignorance, which is the usual basis of probability. If I toss a coin once and it is concealed in my palm, before I reveal it, there’s a 50/50 probability of tails or heads even if it’s the only coin toss that will ever be made in the history of the universe. That’s because the unknown physical factors that lead to one result or the other are balanced and don’t favor one result over the other. I am ignorant of these factors so I assign a “probability”, even though of course the result has been determined. I feel like this particular “argument” is an example of just how distorted reason can be by one’s prejudices. If the authors of the argument weren’t determined to prove MWI inconsistent they would never have conceived of it.