I mean, I don't think we disagree on that, but that's not the reason I'm voting to close.
If I looked at the stats as-is, I could say "oh, that's a CR 5 creature: tons of hit points, but 0 damage output, so we'll give it a 5 just to accommodate the possibility that the environment is a potential hazard".
I could also say "CR25, because this spellcaster has access to Meteor Swarm, and with a save DC of 24, that would TPK most parties that aren't already level 15+, and anything on top of that would be lethal to most parties".