I don't think this is a good principle to apply comprehensively. The "if you’re often over-committed or too scattered" qualifier is very important. Sometimes you need to get out of your comfort zone, which absolutely means saying a timid "yes" to something when you would much rather say "no."

Another framing is "Prioritize". It's not yes or no; it's what are you focusing on right now. There may be plenty of good ideas that are "no" today because you can only work on one at a time.