If the government has a high-resolution camera and the latest tech maybe.. But a regular security camera would probably need at least eyes plus the nose/mouth to make a facial model afaik. If a person enters their home station wearing a mask, and returns with a mask later, that probably keeps them quite anonymous.

A more immediate concern for the protesters is probably that police were stopping young people clad in black in MTR stations at hotspots, as well as those taking minibuses leaving the scene of the June 1 event, and jotting down their IDs.