This is not the same explanation that I was familiar with. The one I heard was that long ago boats didn't have the rudder on the centerline, but was to one side. Because of this configuration you also had to have the dock on the other side. The rudder was called the a steer-board (eventually pronounced starboard). And the dock was the port so that became the port side. So now we have starboard and port sides of the ship from this old boat type.

Reminds me a similar bit about some dimension of the space shuttle being limited by the width of a horse's ass [0].

[0] https://dwanethomas.com/roman-chariots-and-the-space-shuttle...