Your reasoning is invalid, because in order to talk about updating your beliefs in this context, you need a metaphysical framework which knows how to deal with anthropic probabilities (e.g. it should be able to answer puzzles in the vein of the anthropic trilemma according to some coherent, well-defined mathematical rules). IBP is such a framework, but you haven’t proposed any alternative, not to mention an argument for why that alternative is superior.
Your reasoning is invalid, because in order to talk about updating your beliefs in this context, you need a metaphysical framework which knows how to deal with anthropic probabilities (e.g. it should be able to answer puzzles in the vein of the anthropic trilemma according to some coherent, well-defined mathematical rules). IBP is such a framework, but you haven’t proposed any alternative, not to mention an argument for why that alternative is superior.