While his attitude might have been poor in response to it, I don't think it's fair to say it was necessarily stupid that he assumed Altaria was Natural Cure (also consider that your opponent there is currently #4 on the OU ladder). Natural Cure sees way more usage than Cloud Nine according to usage stats too (like roughly 80/20 ratio). Cloud Nine is definitely useful, but given that burns ruin DD Altaria sets, Natural Cure is a very fair assumption.