"In-group preference" is where a person is preferential towards people of their own group or demographic.
Historically, virtually every demographic has been highly incentivized to have a strong in-group preference for survival, though more recently certain groups have been disincentivized from having in-group preference through demoralization campaigns and social pressure.
Groups that are disincentivized from having in-group preference quickly become terminal, as they become self-defeating and disinterested in their own survival.
In my opinion, $ETH suffers from this. Last cycle, ETH maximalism was the de facto stance for most ETH and DeFi users. Now ETH maximalism (even from ETH maximalists) is seen as cringeworthy (see also: Bankless) and ETH maximalists are often more likely to get social support for putting ETH down than for trying to publicly build up it up.
BTC, however, doesn't suffer from this at all. BTC in group preference remains remarkably strong and socially incentivized within BTC circles.