This video is pretty stupid honestly, very much grabbing for straws.
Did you actually perform the mental experiment of sunlight reflecting off the water on a spherical Earth vs a flat Earth? The reflection would work pretty much the same, although the Sun eventually sinks below the horizon on GE, while it wouldn't disappear on a FE.
There are inconsistent explanations for the Sun actually setting on flat Earth which involve visibility and light attenuation, however this has never been formalized in a congruent model, obviously, because it would require different explanations for every different sunset.
On a globe earth you are on one side of a bump.
The sun is on the other side of the bump.
The light reflection on the water cannot go up the side of the bump over the top and down to your feet.
There might be a short reflection of light but it would not be able to extend all the way to you.
He showed it very well in the video.
Here is a drawing from Dingbat. (I hope she doesn't mind me using it in this thread.) She was using it to explain another aspect but for our purposes just consider the first example.
You can see that the light reflection on the water would not be able to go up and over the bump of the curve and down the other side to the observer's feet.
