Of course, my mistake. I read the Norse part of the book on an IcelandAir flight with layover in Reykjavik, which created a nice feedback between what you read and what you see through the window, and evidently got the two mixed up in my posting.
But whether we call the Greenland Norse a civilization or not is really besides the point. Diamond uses it (and other island societies) as a "case study" for comparative purposes, which is almost an ideal application of the Mill's methods of inductive reasoning. Whether we call this case a "civilization" or an "outpost" is really quite tangential for Diamond's argument, which imho is quite convincing. What really matters here is that (i) the social unit in question was relatively small and isolated, which allowed analytical separation of various conditions of interest; and (ii) it was located in a fragile and unforgiving environment where threats to sustainability manifested itself quite quickly, which allows the observation of the effect within a relatively short time period.
Wojtek