Demerits of oscilloscope performing measurement and math functions on displayed data
I had to mess with some Keysight and Rigol equipment and came across the same thing, though I thought Keysight did look at all the data and Rigol didn't so maybe there are some Keysight models that look at all data, not just displayed. Tektronix's version is preferred, though Keysight makes pretty good equipment and probably shouldn't lag behind too much for most applications. I had one of the newer models on loan, so perhaps they have improved their equipment.

I cannot recall if it is was just the Rigol or also the Keysight, but I think the screen resolution actually did matter a lot on measurements and could become a hindrance when the measurement functions only used the display data. With a low resolution (when zoomed out), the measurement function had less dynamic range to work with if I recall, which ended up affecting things when trying to view an ADC's output and voltage steps. Not good, but wasn't critical for my purposes. I would say the video does show some pretty bad issues with the Keysight though, that was quite a bit of fluctuation.