Should be OK. I suspect it is going to be a relatively loose consistency, dropping off spoon rather than sticking if held vertically. If you know what I mean?!?!
from reading the recipe, I would say yes it will set, he says it will come together in the fridge and to "ladle" it into the serving dishes. I reckon the illustration shows a portion that was set in a big dish then divided into smaller dishes to serve, hence it looks more set.
there is a recipe o the back of swiss miss hot chocolate that is wonderful, and if you need some qwik, then get frozen chocolate coolwhip, and set it out of the frezzer for 10 minutes, and enjoy