What if every customer wants two toppings added (all wishes are + +)?
The graph has edges like NOT x_i → x_j for various i and j. You cannot reach x_i when starting at NOT x_i because all edges go toward positive nodes. There is no path backward. Every variable and its negation are guaranteed to be in different SCCs. Always satisfiable. The assignment can be any valid topological choice. Often you can set all toppings to true and it works.