Steelmanning this, there is one interpretation where the probability would indeed be 0.5, namely that every universe only ever simulates exactly one other universe. This would entail that there is always exactly one simulated universe that does not contain a simulated universe.
However, there is no reason why any universe would be limited to simulating one universe, so the interpretation doesn't make sense. The level of branching determines how many 'leaf' universes there are and thus the probability of being in a leaf universe or in the root universe.
I was referring to an explanation I heard, in the context of our own existence in this universe, where that explanation, in my opinion covers a 50-50 chance.
However, there is no reason why any universe would be limited to simulating one universe, so the interpretation doesn't make sense. The level of branching determines how many 'leaf' universes there are and thus the probability of being in a leaf universe or in the root universe.