Yeah. It's true. It dissolves the fondant because its got such a high moisture content. You can decorate with buttercream on whipped topping....provided the buttercream decor isnt too heavy on the sides of the cake. You may be limited to drawing on a design with buttercream on a whipped topping cake.
Now, Ive used Rich's Bettercreme as a filling in fondant cakes before. BUT you have to have a really thick dam of buttercream so that none of it leaks out and touches the fondant. And of course the outside of the cake has to be frosted in buttercream before you apply the fondant.
I dont do this with real whipped cream frostings though, because I hate refriderating fondant. The condensation makes the cakes look horrible. Had an issue with my son's b-day cake actually. It had cream cheese filling. The fondant condensated so bad (humid and hot here in FL) that it made the fondant sticky and shiney and it didnt dry properly even after several hours. I normally make a rule not to refridgerate fondant cakes, however it was for my son, not a customer, so I didnt really care too much.