I am not a young earth advocate, nor have I read ICR stuff in a while, but I remember reading Henry Morris's book long long ago, and I don't remember that being true. Have things changed in that camp?
Creationists commonly claim all strata from Precambrian times all the way up to into the Tertiary are Flood strata. If they’re claiming differently for the Grand Canyon, they’re contradicting themselves and creating a problem—those are fossiliferous strata, so when were they laid down if not in the Flood? (Non-catastrophic fossilization is verboten.)