The Prius uses cameras not radar, and I've driven under hundreds of overpasses without incident.

More likely someone confusing the brake warning with the auto-braking shutdown warning. Because it uses optical cameras, driving into bright sunlight or into deep shadows can cause the system to shut down momentarily, and it beeps to let you know.

Even best case scenario that tweet has nothing to do with this incident because the technology and systems are different. Worst case it never applied the brakes at all, and the driver got confused.