Champagne is not the same gene that produces a palomino.
So if the horse does indeed have the champagne gene, then it is not a cremello.
I don’t know why the AQHA doesn’t recognize the champagne gene, especially in this age of genomics. A DNA test can easily identify the champagne gene, and distinguish it from the cream gene (that creates palomino and buckskin, cremello and perlino).
The complications arise from the names that you would call the particular colors. Since, for example, the cream gene is independent of the dun gene, you can have a horse with both dilution genes. Add the champagne gene to that and you can have some wild combinations… Champagne dunalino, anyone?