What does it mean that saying no is boundary enforcement?

Every boundary is enforced through the word 'no.' If you cannot say no, you do not have boundaries — you have preferences that anyone can override.

Example: Your manager asks you to take on a third concurrent project. You have a boundary: no more than two active projects at once, because beyond two your quality degrades and your sleep suffers. You say: 'I want to do great work on what I've committed to. I can take this on when one of my current projects ships — would the 15th work?' You just enforced a boundary. You said no to the request while saying yes to the relationship and to your own standard of work.

Try this: Identify one request you said yes to in the last week that you wished you had declined. Write down: (1) what you actually wanted to say, (2) what stopped you from saying it, and (3) one sentence you could have used instead. Practice saying that sentence out loud three times. Notice how the discomfort decreases with each repetition.