Opinions are simply opinions until proven fact!
If that were the case just because it is dominant, dilute, and color modifying then colors like buckskins, palominos, duns, tobianos (to name a few) wouldn't let gray override them either. Creme and dun are dominant dilutes for sure, so they are in the same boat as champagne.
If champagne were invincible to grey, then that horse would not be the shade of grey it is now. The horse's coat will eventually grey out white.. (whether or not it gets there before it dies, who knows.. the greying process can be varying speeds on specific breeds -- like Percherons it tends to take a longer time whereas Andalusians will usually grey out to white pretty quickly) ..however the freckles of the muzzle and on the privates will stay there. And since gray is also a dominant gene, that may be why there is so much coat freckling and the horse appears to still have color... just looks like champagne and gray had a little fight on who gets to shade the horse! (But of course in the end grey wins/will win..
)
I'm not too familiar with any of their[scientists] progress with the gene, and just know the basics I suppose, but the champagne not letting grey override theory has just been disproven by those photos. So, I think I may be able to safely say... that the people with those opinions of champagne not letting grey take over are WRONG!