My experience with user testing is that if you give someone a UI and watch what they do with it (without instruction), you will quickly get a feel for how close to the mark your design is. Different users have different expectations of course, so you need to do this with a fair range of users.
However, I do think there is some merit in getting those in the actual field of functional and "good" design (as the author defines it) to weigh in on why precisely your design is just that. In my own UI/UX research, users all too often miss fully comprehending the painstaking research and work that goes into truly effective and good design.
I guess what my argument really is is that I would prefer to weigh more heavily the affirmation of those in the design field rather than a broad range of users with no particular ethos concerning design. Of course, who we end up designing for is the normal user, and what they think is indeed important. However, if 100 people said my design was good vs. hearing that from, say, Mike Matas, I would pick Mike Matas.
Your target is the users, if the users aren't having a good experience, you're not doing it right.
Testing on your target market is pretty important because you get a "real" example of how usable your product is. For example if you tested your interface on an elderly person (target market) and a UI expert you are going to have considerably different results.
Rule no.3 stating that a group of designers must decide if a design is good or not is far more relevant to what actually is good design, verses the general populace.
That's like asking a group of patients if they think the drug that's being made seems good enough with several side effects or not, verses asking pharmaceutical experts on the matter.