This advice is probably well intentioned, but is diametrically opposed to allowing people to clearly set boundaries. Sometimes, a direct "No." is the healthy response if someone asks you to cross a personal boundary. Where to eat lunch isn't a great example of such a case, but disallowing people to say "No" is a poor precedent to set between people.
True, but I think his point is that where there's a negotiation happening (which is what deciding where to eat is), just saying "no" isn't that helpful. More helpful is to say why you're saying "no" so that the other people can formulate an alternative proposal that resolves your objection. That back-and-forth is the essence of a successful negotiation.
Boundary-setting is something entirely different in that it's not about negotiations. In those situations, a plain "no" is often the best thing.
