That's the problem with time travel. You can't really keep it a secret. The mechanics are based on the laws of nature in theTrek universe and because of that, will be eventually discovered by other races. Or enemy spies will manage to steal that information. It's like the atom bomb: once the genie is out of the bottle, you can't put it back in. It also raises an interesting question. Why there hasn't been any more attempts to mess up with Federation's history? Those temporal agents must really have their work cut out for them. I would think that sort of stuff would be an everyday occurance.