Consider that the Pogo 30 has a beam of 3.7m while the Winner 900 have only about 3.0m. This means that the overall surface to be built is considerably smaller and therefore less heavier. I don't know where you get the information that the hull was not cored. Here they show a cored hull being built
I guess you can read German. There are German test on the net on the Winner 900, but it was not the performance version:
My information about the hull was: Winneryachts - Way Ahead - Building a Winner
Which I clearly misunderstood, as also confirmed by the Yacht test (my translation):
Hull and deck are built in sandwich with hand lay-up.
Less beam = less hull = less material = less weight.
Plus 300 kg for the interior now also makes some sense to me.
Thanks Paulo, my honest mistake!