The authority that Duane cited said "it all depends on displacement." If we assume that is true, just for the sake of argument, then the one quality that distinguishes a heavy displacement boat from a light displacement boat is that the heavy boat moves more water out of its way than the latter. If the hull has to move more water out of its way, more energy is used up to drive the hull through the water. Perhaps a light displacement boat is capable of pushing its own, smaller, bow wave through the water at a faster speed than a heavy displacement boat can push its bigger bow wave through the water, using the same amount of energy. I don''t know whether this reasoning is correct, but it seems to be consistent with what you and Duane''s authority are both saying.
Also, Duane''s authority says, "Everyone is familiar with Anthony Deane''s original formula for heavy displacement hulls, and people are slow to catch that non-planing boats go faster than Deane''s formula predicts, despite our observations that boats sometimes do go faster than they''re supposed to." You said, "Waterline''s affect on hull speed is theoretical and not absolute." Is this an indication that, as boat design becomes more sophisticated, the authorities are coming to the conclusion that Deane''s formula should only be taken as an approximation, and that, with future advances in boat design, the upper limits of speed can be raised, and that perhaps Deane''s formula itself should be modified to reflect current technology and thinking? This would certainly explain the huge amount of anecdotal evidence that boats do in fact exceed the speeds predicted by Deane''s formula, without planing.