In general, I’d agree with you about shorter brush enabling faster forward motion, but when the wind is up like this, the flame front gets blown right across the top.
After ignition is when the height and density of the brush comes into play because it determines how “deep”(thick) the flame front gets (taller brush, “deeper”/thicker flame front), which influences how hot the it gets (thicker front, hotter peak temp), which determines how far in advance of the main body of flames the heatwave can be pushed by the winds and ignite combustible materials.