mdd – the mitteler did not admonish people for sleeping in the sukkah. the way everyone else has the story, his statement of “how can I sleep….” was said regarding himself, to explain his own practice of not sleeping in a sukkah.
And parroting the lubavticher rebbe’s kasha/terutz of “chabad is so strict about achilah so why are they not strict about sleeping” – is just a narrative. I’m not beholden to his shakla vetaryah. I view it differently; to me, chabad were no different than the alfei revavos of yidden who did not sleep in a sukkah in europe, either because of the cold, or because their wives weren’t comfortable(both are mentioned in the Rema as heterim)
So if they want to add kabalistic significance to an established heter – chasidim do that all the time, so be it, but no one besides neo-chabad takes kabalah and uses it to overturn halacha.