There has been talk of this here:
Not sure if you can make it output horizontal field of view, you would have to use a trigonometry table to find the equivalent horizontal angle given the vertical at a certain aspect ratio? or use advanced camera and figure it from the lens info.
As mentioned, the horizontal is usually the dominant angle in cinematography, which maybe why google said as much? as it maximizes the film sensor’s width and depending on aspect ratio will crop the top and bottom and avoid vignetting if a spherical lens were only covering the height of a sensor. To be fully covered it would be the diagonal measurement.