Everyone thinks he/she is an expert when they know nothing about which they are offering a critique. When minds are full of inaccurate information it is impossible to offer these individuals the correct information. They need to "empty" their heads of their bad info in order to "get it"! This could be a monumental challenge and personally I'd rather not waste my time giving them "the right stuff". You did the right thing - you were polite and shrugged it off.