GRP (glass reinforced plastic) is very strong in tension but not in compression. Thin washers under nuts torqued to pretty high values don’t provide much support especially if the holes are a little oversized to accept the location tolerances of the studs in the keel. Since stress=F/A you want to maximize “A” to minimize the compressive stress on the GRP. Thus backing plates versus washers.
The wood core theory is valid but a core sample might be needed to test it. Wood is not that strong in compression either if you ever tightened a nut and washer on a piece of plywood it is easy to embed the washer into the wood.