Good points.So, I think people are in a spectrum of all of this, and that it's important to recognize where our default tendencies lean towards and in which kinds of situations, so as to better adjust them. It's about finding more balance, and taking the law of three into account. (The is right and wrong - yes and no-, and the specific situation). Trying new approaches can be useful too (often to discover that what we anticipated would happen doesn't happen at all!)
IOW, a "no" can be just as mechanical and emotionally driven as a "yes", so it's about becoming more conscious, knowing our limits, our values, our goals, and respecting those a bit more, while doing our best to respect those same things in others.
Anyway, just some thoughts on the matter, but maybe the author already covers these in the book. So, FWIW.
I think a person can say "No" in an agreeable way and say "yes" even when some part of them doesn't want to, as long as they are conscious of what they are doing and why; it's called "external considering." The important part is to be conscious.
But for many people - and I go through it myself from time to time - just being able to say "no" at all is a big hurdle. So learning how to do it and figuring out that you won't die from it comes first; being conscious of being able to say "yes" or "no" depending on the specific situation can come after.