Using Sterno to make S Mores with is perfectly safe as the Sterno is actually a pretty clean burning cooking fuel.
I cook and make S Mores a lot when camping over the Sterno cooking fuel and never have I gotten sick.
I actually asked someone at the Sterno company about this exact thing and they did say that the Sterno burns clean enough that it's very safe to cook food directly over.
So they are correct as I've never gotten sick nor tasted any residue from the Sterno fuel when making S Mores.
I use Sterno cooking fuel a lot for cooking with and cook hot dogs as well as the S Mores on and it never has been a problem.
I do find that Sterno does burn really clean and is a good efficient cooking fuel and is safe to cook food directly over.