Basically, he filled the hole with fiberglass/resin, put a thin layer of grease on the bolt, and stuck it into the fiberglass/resin. Once the resin hardened, he could unscrew the bolt (the resin would have flowed around the threads, but the grease would have kept it from bonding to the bolt). Then he put the bolt through the seat, and screwed it into the threads in the fiberglass/resin that he just created.
I did similar, but used epoxy with a brass insert
embedded. That gives a very strong set of threads to screw into, and a much bigger diameter that grips the epoxy.