You've confused kin selection with kin discrimination. The idea "because friends are genetically similar, an individual with the opposite genotype at a specific allele poses a bigger threat to your survival/fitness than someone with the same genotype at that allele."
Additionally:
> formal analysis has shown that
selection for group adaptations requires special circumstances,
with negligible within group selection (Fig. 4), such
as when (a) the group is composed of genetically identical
individuals (clonal groups, r=1), or (b) there is complete
repression of competition between groups (i.e., no conflict
within groups; Gardner & Grafen, 2009).
It is useful here to distinguish adaptation and design from
dynamics of how selection leads to design. The dynamics of
selection can be examined with either an individual
(inclusive fitness or kin selection) or group selection
approach. However, only the individual level approach
provides a general model of adaptation. The idea that
individuals strive to maximise their inclusive fitness holds
irrespective of the intensity of selection operating within and
between groups (Section 2; Fig. 4). In contrast, as discussed
above, group adaptations or maximization of fitness at the
group level are only expected in the extreme case where
there is no within group selection.
I really doubt your view is consistent with the conditions for selection in the group. If non-tribe-member competition caused significant competition between groups, then it far outweighs the pressure of in-group adaptation. If "friends are genetically similar" is a genetic adaptation from tribalism, then it has to be an enhancement from within the group when it lacks competition - you benefit from the adaptation relative to people within your tribe that don't have it. FYI the paper disagrees with the notion that friend similarity is likely a result of decision.
Personally I think that 1% relative similarity could easily be explained in the ambiguous language they use to explain why it was "mostly" homogenously European - why would it surprise me that the non-Europeans would tend to have less DNA in common with the friends they have while also likely having fewer friends per capita?
Though, possibly, because the most likely to be similar SNPs were related to smell (and therefore taste), and to linoleic acid, maybe it reaffirms that people who break you bread are your companions - and that there is a real difference between butter cultures and olive oil cultures :P
[0] https://www.zoo.ox.ac.uk/group/west/pdf/WestElMoudenGardner_...