Do you believe that because a movie has Godzilla that nothing else can work the same as the real world?
What do you mean "work the same as the real world"?
Fictions that involve departures from what is possible in the real world necessarily entail that, in those fictional worlds, the rules that govern our real world don't apply. Whether there are other possible rules that explain the fictional world is a further question. Generally, I don't expect the fiction to offer such rules - rather, I expect it to gloss over the inconsistencies and impossibilities.
Thus, if a movie has Godzilla in it, then I would expect it
not to also include an elephant falling a couple of metres and breaking a limb as a result - because that would draw the audience's attention to Godzilla's impossible biodynamics.
More generally, I would expect the movie to handwave any biochemistry or physiology - again, to avoid drawing the audience's attention to the nonsense that is Godzilla.
To come back to D&D, it is full of impossible animals doing impossible things. That is enough to show that it does not treat actual biochemistry, biodynamics, physiology and fluid mechanics of flight, etc as default assumptions.
Or to put it more simply: in D&D, both birds and dragons fly. That's an obvious truth of the gameworld. Bernoulli's equation can't explain the dragon's flight, and so it makes no sense to suppose that it is nevertheless the default explanation for the bird's flight. The worlds of D&D
have not scientific explanation of how animals fly - they just
do, in the same way that mortals just
have souls that can survive their bodies, contain their thoughts and memories, etc.