In the cases where it is used in a gender-neutral but expressly singular sense, this is false. In cases where whether the number of the referent is ambiguous and might be either singular or plural, the use of the plural they has been acceptable longer, and has never been objected to even by the prescriptives who have tried (and are still trying, though their influence has waned somewhat from its late 19th Century height) to force generic he in place of singular they as part of their effort to impose Latin-inspired grammar rules on English.