I can’t believe this was an issue. They just now figured out that currents can impact the wave. I must be missing something but this sounds too basic based on what I just read. Maybe the author doesn’t really understand what he wrote about so it’s not clear.
From my recollection, the famous Michelson-Morley experiment was designed to detect the "medium" through which light waves move in a vacuum. If the earth was moving through such a medium, then the measured speed of light would depend upon which direction the light was moving relative to the direction of earth's movement.
Much to the surprise of many, there was no detected change in the speed of light despite the known movements of the earth.
The expectation that there had to be a "medium" in which light waves occur was consistent with many physical wave phenomena including waves in water. For this reason, I too would be surprised to hear that wave behavior in moving water would be unstudied until now.