As a professional lighting technician, I can affirm that this is incorrect. Incident meters are designed to measure the intensity of light at the point of the sensor, whether your source is broad and diffused or narrowly focussed.

What you really need to judge a headlight's effectiveness (electrical and durability considerations aside) are two pieces of data: beam characteristics (shape, falloff, etc) and intensity. Beam pattern photos at a standard distance and all shot at identical exposures are a good way to judge the former, and an incident lighting meter that gives you readings in footcandles is an excellent way to judge the latter. A good set of data might be to measure the distance from the fixture at which you get 2 footcandles in the brightest part of the beam.