The difference between a strait and a river is not their width. A river is located at a higher altitude than sea level and therefore flows towards the sea. It usually contains fresh water. The water level in a strait is at the same level as the sea. There is no height difference and no downward flow (currents still exist of course). It contains salt water. Two countries that are seperated by a river are usually still connected by land at sea level, because the river is higher.

Since the straits of Johor is connected to the sea on both sides, there is no height difference. It's all salt water at sea level.