So the likes of Joshua Wong are still safe?
No, he won't be. Since the enactment of the National Security Law would mean that Hong Kong will have its own domestic National Security Law (To be implemented by Hong Kong Authorities), the Hong Kong Police, judicial and penal system will then have a law from which to enforce the legislation. i.e the HK police will then have the legal backing to arrest the likes of Joshua Wong on the basis of being a national security threat such as promoting HK independence (secession), taking money from and actively engaging with foreign powers to further foreign interests.
This is under the assumption that he is in HK at the time. He may (or may already have) seek asylum abroad in the Five Eyes counties who certainly would not extradite him under "Human Rights" reasoning.