There is nothing wrong with the logic here. It doesn't mean that "Lenin was a bottle nose dolphin" would have been more plausible - it simply means that a different argument would be used to refute it (such as: "a human can't be a dolphin").
E.g., imagine there actually existed some little-known mammal/plant hybrids. The "historian" could have presented one and dramatically refuted the argument. Of course that wouldn't actually help in proving Lenin was a mushroom, but it would let that proposition appear more credible.
If you see a helicopter in the sky, but somehow believe it is an airplane, saying "it can't be an airplane because it has no wings" is perfectly valid. It doesn't somehow make the claim "a bird is an airplane because they both have wings" more plausible - it just means that a different argument is needed to refute it.
Well, I think, even if that were the case, it would still be a logical fallacy, because the fact that a mammal could be a plant/fungus/whatever doesn't tell anything about whether that particular mammal is. All it does is to increase the "refuted counter arguments" score which gives it the illusion of credibility. So the whole "can a mammal be..." debate would be a red herring.
You could pull the same trick with the airplane example: "What do you mean, this thing can't be a plane because a plane needs wings? The rocket plane XYZ-34 doesn't have wings either." (while ignoring the fact that the helicopter you're watching doesn't look anything like that rocket plane)