Well, the Wisdom is 1 because they are completely mad. While it does not have much sense mechanically (the role of ability scores in monsters is quite obscure in this edition), I wanted to do it for the fluff. That said, I think I will give it proficiency in Wis savings, so the malus reduces to -1 and we avoid the scenario you describe, which admittedly is quite absurd. The Remorhaz has a +0 bonus and is a CR 11 as well, so it should be on par.