I wonder whether he understood you meant the pure, essential lemon oil that is recommended here? Buy a bottle of the stuff, and compare it to mineral oil yourself - I believe you'll find that the mineral oil is clearly thicker and more slimy feeling.
Aside from the fact that it sounds backward, I'm not sure I understand the viscosity argument anyway, and don't think there would be anything particularly wrong about using mineral oil.
Perhaps he misspoke, and was concerned that the thinner lemon oil would soak in more easily, and that in a dry climate you might be tempted to overdo it. But as long as you're not trying to saturate the thing, I don't think you'll have a problem with either one.