I-28 Keel stub construction?
I believe that "sealant" is thickened resin which the builders probably mixed up and poured in the stub as a leveling agent / bedding compound. On top of that thickened resin, they "float" the plate which is cured into the hull. An extra step with better materials for a great boat...
If we cut the boat in half, top to bottom, I would expect to see the hull layup runs down the sides of the stub and are folded across its bottom. The plate provides an excellent loading point for the nuts of keel studs. Between the plate and the keel, the "bottom" of the stub distributes the loads to the hull.
If you get the opportunity to ride a few examples of i28, when driving hard to weather lift the cabin sole hatch, sit down and watch the stub work. An i28 I ride which has been raced summers for 25+ years, flexes to the degree it is easily noticeable to the eye. Another much less so and ours, since replacing the floors, very little. Not "rock solid" however as I think it needs to flex a bit to "save itself" from overloading the next link in the chain...