"The difference between successful people and really successful people is that really successful people say no to almost everything."
That's very different. This is a message that is useful to hear for extremely driven ambitious people who are already successful - that it's better to spend all the time you spend working on a few carefully chosen things. It should not be interpreted as advice for people who are not extremely driven to not do things. The baseline assumption is that you already spend a substantial amount of time working on potentially productive things.