In common usage (based on a Google search for “all * are not *”) you are wrong: in fact, most usages of the phrase seem to mean “not all X are Y”. Probably the phrase is ambiguous, but then we should not use it at all, and either say “No X are Y” or “Not all X are Y”. And in that case it is silly to criticize a use of the phrase which you admit that you have correctly parsed.
Most people also understand “if” to mean “if and only if”; it does not follow that we ought not to correct such ambiguous and context-dependent use. I’m down with common usage in most cases, but not when it comes to making logical distinctions in writing. There is a place for prescriptivist precision in language, and this is it.
In common usage (based on a Google search for “all * are not *”) you are wrong: in fact, most usages of the phrase seem to mean “not all X are Y”. Probably the phrase is ambiguous, but then we should not use it at all, and either say “No X are Y” or “Not all X are Y”. And in that case it is silly to criticize a use of the phrase which you admit that you have correctly parsed.
Most people also understand “if” to mean “if and only if”; it does not follow that we ought not to correct such ambiguous and context-dependent use. I’m down with common usage in most cases, but not when it comes to making logical distinctions in writing. There is a place for prescriptivist precision in language, and this is it.