I think most people think l-zombies and p-zombies are impossible

I doubt this holds even for LW regulars. (I feel confident it does not hold for all visitors.)

To my way of thinking, we have no reason to believe in a mathematical macrocosm. Naive Tegmark 4 gives false predictions. Now this could reflect our confusion about anthropic probabilities or "reality fluid", but the same confusion could make Tegmark 4 seem more attractive than it actually is. (I don't think that last point applies to MWI, which I more-or-less believe because it seems like the natural reading of a successful theory.)