People who as their first reaction start pulling excuses why this must be wrong out >of their asses get big negative points on this rationality test.

Well, if people are absolutely, definitely rejecting the possibility that this might ever be true, without looking at the data, then they are indeed probably professing a tribal belief.

However, if they are merely describing reasons why they find this result "unlikely", then I'm not sure there's anything wrong with that. They're simply expressing that their prior for "Communist economies did no worse than capitalist economies" is, all other things being equal, lower than .5.

There are several non-obviously-wrong reasons why one could reasonably put a low prior on this belief. The most obvious is the fact that when the wall fell down, economic migration went from East to West, not the other way round (East-West Germany being the most dramatic example).

Of course, this should not preclude a look at the hard data. Reality is full of surprises, and casual musings often miss important points. So again, saying "this just can't be so" and refusing to look at the data (which I presume is what you had in mind) is indeed probably tribal. Saying "hmmm, I'd be surprised if it were so" seems quite reasonable to me. Maybe I'm just tribalised beyond hope.