It's a single bridge, going over water and underground, into an area that requires blue water presence. Choke points work both ways, easy to choke but also easy to secure. The permanent garrison of PLA in Hong Kong would be able to secure the HK side of the bridge long enough for reinforcements to get through.
China's biggest worry in this area is civil unrest, not hostile foreign powers and most of the bridge is inaccessible to potential rioters or dissidents. It's a huge game changer for the region, allowing China to move initial response troops into the area much faster. (30 min vs 4 hours)