Engine seized

Neither. The cylinder head gasket developed a leak, which is not uncommon in any car, but the coolant caused "hydro-lock". Liquid can not be compressed, so the sudden gasket failure filled that cylinder with liquid coolant that stopped that piston in its tracks. With the high momentum of the spinning crankshaft, something had to give, and that is usually the connecting rod. The proof is the bent connecting rod. That tells us this occurred unusually quickly and most likely at highway speed. This has nothing to do with the timing belt.

The glaring clue here is the 600 km. If something was done incorrectly with the timing belt, that would have shown up right away, and it would not have damaged the connecting rod. Sometimes engine failures can be hard to figure out what happened first, but this one is pretty straight-forward. It is unfortunate, but it can happen on any brand of car.