When I said that they “wouldn't be able to swim or to walk” I meant that before the fin became a viable leg it would make a poor fin; therefore the creature would neither be able to walk nor swim.
Sea turtles have fins which work quite well for swimming, and yet allow them to walk on land for brief periods.
Other turtles have fins that have developed proper feet, and can walk on land much better, yet can still swim effectively.
A "poor fin" or "poor leg" can still be "good enough" for a creature to survive long enough to reproduce, which is all that evolution requires. Nobody reasonably asks for a fish to suddenly spring a real leg that is fully useful on land. We expect intermediate forms, of which we have a large number of examples.
Every time a fundy breaks the laws of thermodynamics, Schroedinger probably kills his cat.
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