I happened to see the original in the Tanchuma today – the idea is totally different: sukkos is the finish to Yom Kippur, for on Yom Kippur Hashem forgives us but we are not yet friends. That happens on Sukkos.
Then forgiveness is complete. Hashem says ‘Ok, its a new account from now on’
So its not discussing the sins of the interim, rather that Sukkos marks the end of the atonement cycle.
What of the interim sins? Could be that they count towards the new account. Its not the subject at all.
I understand that the Midrash Rabbah has it a bit differently, that we are too busy to sin. In that case the other ideas expressed here hold.