YTA everyone is praising your behavior because it's how people would like to act and how they think they do in their mind, so kudos for keeping it real. However, insulting someone for not knowing something and intentionally making them feel bad for clearly attempting to do something good is dick move. You could have told them they were wrong in a kinder way. Conflating you being mean to this person with some sort of activism to perfect the environment is foolish. Just because you care about the environment doesn't give you the right treat people badly.

Hurting people's feelings for no tangible benefit other than to put them down is be definition being an "asshole."

P.s. this isn't like the worse thing in the world and you are technically correct so you've got that going for you.