From the text I think he draws a distinction between consciously deciding not to do something and missing it entirely.
Level 5 allows consciously dropping — saying “I’m sorry [can’t do it] a lot”
Level 6 - you aren’t seeing everything and they are getting dropped WITHOUT intent. “Just saying “No” to inbound things is not enough.”
As an aside, guidelines violating clickbait number title? “The Seven Levels of Busy”
At many workplaces, it's totally OK to drop things deliberately. I've been told by managers that [Company] deliberately loads you with more than you can do and expects you to make the right judgment calls. It's even sometimes OK to drop them by accident as long as you know they are being dropped and communicate the droppage after the fact. When you start dropping things and don't even know you're dropping them, that's usually where it starts becoming a problem for the organization and where it starts coming up in performance reviews.