The cheese and onion smash has syns according to syns online. Normal smash is free but counts as a tweak when used in a way other than intended, so as ingredient like that rather than as actual potato.
Some can tweak to their hearts content and get away with it. Others can't. You could make it and not syn it and see how you go it you can use the 20kcal per 2 syn rule. I've never eaten smash so I can't comment on how she worked it out and whether she used that rule or not but for a guess that's what it works out as when synning it