I have heard this story before and it seems plausible enough. (Stairway To Heaven maybe gets a little too frantic in its final quarter to be considered a ballad per se)
On the other hand, it is known that Jimmy page still doesn't like Led Zep being branded with the all too easy label of Heavy Metal. He's very proud of the fact that a full third of their output was acoustic music, the influence of Bert Jansch and Davey Graham is regularly to be heard on their Guitar work and they had already utilised Sandy Denny as a guest vocalist. All pointing to a strong, ongoing folk influence on the group. That, however, does not disprove the claim and as far as I am aware no involved party has ever disputed it.
Nice one, George.