Even the isshiki's level 4 scale is completely questionable. What would that make him? Millions or billions of times stronger than Peak Kaguya in physical terms, so the Kaguya who left him on the near death was even weaker. The "offguard" argument is ridiculous in the way it's being used, at most it means he can't react. It makes no sense for "offguard" make him billions of times weaker, especially for an Otsutsuki, since as far as we know their chakra remains entirely within their body, they don't need to mold anything. And let's be honest, she didn't just drill a hole in him or something, she COMPLETELY DESTROYED part of his body.