But in Hong Kong's case, a number of Telegram chat room operators have been prosecuted by the police for anti-government activities. If TG is really secure then this shouldn't have happened.
Well there is a number of ways for identity information to leak, from simple self-deanon mistakes to "third degrees" of key positions who were unfortunate to be caught in action and unable to prevent their chats from being analyzed. A good old handcuffing to a radiator still works too. Your claim can only be strictly true in an ideal world of ideal operation on one side and an ideal human-respecting law on the other.