just go out and buy some evaporated milk. it seems as if the ingredient list is calling for evaporated, NOT sweetened condensed milk. In America, if sugar is not added to the process, aka non-sweetened condensed milk, it is referred to as Evaporated Milk. If sugar was added before water was evaporated, then its referred to as Sweetened Condensed Milk. Which, imho, is an excellent ingredient in chocolate recipes. If you wanted a more sweet and creamy versus milk-chocolatey... um... chocolate... then I'd used sweetened condensed. evaporated will get it closer to milk chocolate, and regular milk would screw up the texture and flavor likely. just go to the store, buy a can of evaporated milk, and follow the recipe. you'll thank yourself.
so, in a word, yes. you can ruin the recipe if you used milk from your gallon jug versus using evaporated, as there is MUCH less water content.