The statement made in the question is mentioned in one of the renowned publications.

My contradictions which I have already ruled out as a potential solution:

• As Density decreases Vmcg decreases thus V1 is not limited by Vmcg.

• As Density decreases Vmbe decreases as there's insufficient cooling of the brakes, thus V1 should ideally be limited by Vmbe.

• At lower density the aircraft needs to be at a higher groundspeed to achieve the same CAS, but the ASDA would be limiting factor as it would require a greater stopping distance, this would again limit V1.