In that case, it is really no surprise, with the Siberian seas open for so long, venting heat into the atmosphere. Even then, it is a strong record. I wonder whether we have a new trend where these seas could be open even into December...
I'd agree, and it seems to me not 'whether' but 'when' to me, as to the Siberian Seas being open in December. The Kara (and Chukchi if it counts as Siberian) are already mostly there, and the Laptev has started knocking on the door.
Over the past 25 years or so, the early refreeze (Sept to Dec) has been progressively delayed and now takes place about one month later than it did around 1995 (Charctic graph below). And the Arctic icecap has remained stubbornly wedded to the Canadian coast and Greenland ice sheet during the the minimum each year, so the Siberian side is likely to be the open water side for the foreseeable future.
In 25 years, if current trends continue, December will be the new November in the Arctic (Dec 1 vs Nov 1 NSIDC comparison for this year below). Eyeballing the graph and map, I would guess the Siberian side (Kara, Laptev, ESS, Chukchi) will probably be largely open at the start of Dec within 25 years. If trends accelerate then in less time -- maybe in just a decade. (And yes, I realize that simple linear extrapolations are dicey in these circumstances, but the decadal patterns do seem clear).