Our train cards (Octopus card) are often linked to our bank accounts so that we can use the Automatic Add Value Service (AAVS). The only people travelling to Causeway Bay and from Admiralty at that time would be protestors so it would be fairly trivial to link protestors to individual identities using the cards.

It wouldn't be too hard triangulating the cell towers because we have a huge population density and, hence, cell towers attached to nearly every building. But I feel this data would be harder to get hold of and may catch the hundreds of thousands of "innocent" people who live in those buildings.

Using triangulation to track a single person is one thing. Using it to track a million would be incredibly tedious. Whereas getting a list of everyone who got off at specific stations in specific timeframes is likely a single SQL query.