The worst part of the �minhag� not to sleep in the sukkah is the reasoning given which doesn�t hold water.
The Rebbe’s explanation of the reasoning came years after the minhag was established, as a possible svorah to support why Lubavitchers are so makpid on all details of Sukkah (drinking water, rain, etc.) except for sleeping in the Sukkah.
Similar to the sevaros given by the chassidish poskim for eating outside the Sukkah on Shemini Atzeres (but with the aforementioned difference that the Chabad is rooted in Shulchan Aruch).
And the fact that it essentially rejects a chelek of the mitzvah.
Chabad absolutely doesn’t reject that sleeping in the Sukkah is a chelek of the mitzvah, ch”v. The chiddush is only that they accepted the kulah of not sleeping in the Sukkah while rejecting other kulos.
Have you recently read the sicha in its entirety in the original, or are you relying on memory, or worse � hearsay? This question is addressed there.