Steam is a separate category as aroma in hilchot Kashrus. The former, if hot, contacts a pot cover, that cover would take on the dairy or meat status of the food below it. However, when water is heated, only H2O departs from the solution, and the solids are left behind. What you would get on the pot cover would be pure H2O, by which logic the cover should remain the status it was before the steam. However, the rabbi’s did not rule this way. Assuming that Chazal were more astute than we are, why do you suppose was the reason, aside from the obvious one of building fences.