When the 90 centimeter measurement is taken just before the 14 minute mark of the video, shown roughly with the yellow line (I'm not saying this is 90 centimeters where I've marked, I picked arbitrary points that look roughly correct), that would really map incorrectly onto the red vertical line. What they should have been trying to do is take a measurement perpendicular to the water, more like the blue line. The green line shows just how massive the angle difference was, possibly 15 to 20 degrees out, which would lead to a huge error when trying to level it.
Adjusting for the horizon, that's 24° of slope. Cos(24) = 0.91, so about 9% error.