Not entirely true- it's possible to do a next action which is part of two or more projects. For example I might make mushroom soup (next action) because I want to use up some leftover mushrooms in the fridge (project 1) and try out a new recipe for mushroom soup (project 2).
Right. Another example: often my actions for the purpose of exercise also involve another purpose:
having fun, transportation, connecting with other people etc.