My recollection is that it was solid fiberglass there, not cored. Of all the things I have removed from the deck I don't ever remember seeing core - always solid fiberglass. I always assumed that Beneteau used solid fiberglass everywhere they mounted stuff or would expect an owner to mount something (like a second cabin top winch for a spinnaker).
I took the bolts off from underneath, pulled it up less than an inch, used a dremel to slightly chamfer the edge of the hole, then cleaned it out. The threaded part of the pulpit stanchion was probably in the hole a bit but not in the way. Then I formed the butyl in a cone shape around the threaded part and flange of the stanchion (sharp side of the cone facing down), reseated it and cleaned up the butyl that squeezed out.
My problem came when the port/stern pulpit got hung up on a piling in my slip, the tide dropped and the boat was hanging by the pulpit. After that I had regular water in the stern berth under the bed cushions. I eventually traced the leak to the stern-most post (middle of the 3) where the prior bedding had dried up and crumbled under the stress of that incident. I did not remove/rebed the forward-most stanchion of the pulpit - the one that sits in the fitting on the deck.
Good luck. For what I did it was way easier than I thought it would be.