Wow- that’s a serious size core. With 2 metre thick walls, I was thinking the same. The DD350 seems a pretty impressive bit of kit, and is according to makers spec intended for stitch drilling cores- seeing the size of those it could be possible but there would still be complications. The limited depth feed of the drill (500 mm?) means either continuous drilling by adding additional extensions or drilling to max depth and then breaking out a part-core. not at all easy assuming the 2 metres thickness is peppered with any one of Chubb’s famous barrier deflectors- tangbar, spiral matress etc.
my guess is continuous holes drilled with extensions until they broke clean through, but either way it must have been hell of a slog with all the reinforcing.