Smash is free when made up as directed on the packet with the correct amount of water. It's not free when used as a substitute for flour as I imagine it would be in this "recipe".
100g dry powdered mash makes up 600g mashed potato. They both have the same calories but its easy to see which would be the most filling, and SW is based on how long foods fill you up for relative to their calorie content.
When using smash in any other way SW advise synning it at the red day rate of 4.5 syns per 25g powdered mash, which is the same as flour so you'd be as well just using the real thing to be honest xx