I think the hot shot is so you don't have to wash and sterilise after every use cos you're only using boiling water with it x
You’re supposed to put the formula in the rapid cool and the boiling water, then it cooks the whole thing to the correct temp. You don’t need a hot shot as the whole amount will be hot, although Louise’s method I easier to be able to keep flask clean :-)
I have the Tommee Tippee version with the same instructions as yours but I do the hot shot method to be safe. I do the whole process slightly differently to the instructions. I fill the cool flask, put formula in the bottle add a hot shot and shake and then pour the cooled boiled water into the bottle and shake again. Saves a full clean of the cool flask as well!