"Arguments about eating in the sukkah on Shemini Atzeret outside of Israel have a long and somewhat baffling history. While not the only example of practice in opposition to the Shulchan Aruch, it appears to be among the most argued. The gemara, Rambam, the Tur and the Shulchan Aruch, written in many locales, all seem to be as unambiguous as possible in requiring one to eat in the sukkah. The Gaon, incensed by the spreading Chassidic custom to eat outside the sukkah, perhaps lemigdar miltah, went so far as to mandate sleeping in the sukkah on the night of shemini atzeret, in opposition to the Maharil, the Magen Avraham and normative custom."
the Seforim blog: The sukkah on Shemini Atzeret controversy
"The question is why it's not put to rest by the gemara's conclusion: 'Vehilkhisa: yesuvei yasvinan, berukhi lo mevarkhinan'".
Here is a simple approach is that there are really 2 different ways [at least] to parse the conclusion
A "The Halachah is that we MUST sit, but w/o a brachah" - this is the normative interpretation of the Poskim
B "The Halachah is we MAY sit, but w/o a brachah" - meaning that despite the ch'shash of bal tosif we are allowed to sit - so long as we say no brachah.
As opposed to the GRA and the Kitzur SA [who EG require even sleeping in the Sukkah ] - the Derech Hachaim and others insist on using the sukkah ONLY when the absence of the Brachah is clearly manifested.
Thus, those who do NOT sit might indeed be flouting the normative Poskim, nevertheless they CAN be harmonized with this "hilchesa" - which is probably Saboraic anyway.
There are other factors involved here "v'ein kan m'komo."