I agree with it seems apocryphal. I can only imagine interpretations of the statement "invert a tree" that are so trivial as to test nothing (as in "invert the multiplication of A * B" uhh, "B * A"?).
I suppose if I were asked I'd ask for clarification of what was wanted, and either they want someone who asks such questions or both of us leave the room relieved that we won't be working together.
And B-trees are much trickier to get right than binary trees. Anyone who can whiteboard a b-tree from memory without errors is some kind of savant.
I think this whole reverse a b-tree story is apocryphal.