It is not incorrect. Singular they is a perfectly legitimate construction and there's nothing ungrammatical about it. Additionally, if a substantial portion of the population of speakers uses a certain word or construction, then it's no less legitimate than any other. That's how language has always worked, it's defined by usage. What you're calling 'precise language' is language without ambiguities, which simply doesn't exist. Ambiguity is an inevitable part of natural language. You're suggesting that non-native speakers might struggle with it, but as a non-native speaker I can assure you that a pronoun with different meanings depending on context, such as 'they' is an absolute non-issue compared to all the other quirks that natural languages like English have on offer, and always have had.