He isn't right... not at all. 8cm panel he reckons? It was more like 12. Don't know why he thinks it was 1m x 2m, it's clearly more than that. It's also probably Oak, which means it density is 740kg/m3. It totally would have been able to hold them both up.
The plot established that it could not support both of them. However, your point is valid and it is a shame that the prop department didn't do the physics when making this prop to make it visibly obvious that this piece of wood could not possibly support both their weights above the water.
Also, suppose there is a huge marble cherub bolted to the underside of that. We don't know.