His calculations are wrong at least in one respect - he incorrectly assumes that glidepath is calculated to the runway threshold, but in fact it should be calculate to a touchdown zone - usually around 1000-1500 ft from runway threshold.
I correctly assume the glidepath is calculated from the threshold, starting at the published Threshold Crossing Height (TCH) of 48 feet. This is included in the calculations.
correct alt (feet, asl) = tan(a) x distance (feet) + threshold elevation (feet) + TCH (feet), where a = glidepath angle