The argument is more about how to define "good".

All of those sites have something that sets them apart in a way Nielsen himself has mentioned many times: most of their visitors probably do spend a significant amount of time there on a regular basis, so they don't need to follow the same rules as everyone else. Of course, whether those sites are successful because of their questionable aesthetics or despite them is a different question, and whether they might be more successful by some interesting measure if they tried a major change in their design is another question again.

In any case, details that are purely aesthetic -- subjective matters of beauty and personal taste -- don't make a difference to ease of use. That's a tautology, because not mattering to other factors is almost the definition of a detail being merely aesthetic. However, more generally, things like visual design and typography do matter and can make a measurable difference.