I think you're talking past each other. What they're saying is that we have a biologically based aversion to mating with those we have been raised with, and that this aversion has arisen due to evolutionary pressure to avoid incest.

Of course, our body has no way of telling whether someone is a close relative or not, and evolution doesn't "care" why we change our behaviour, as long as the new behaviour gives a higher chance of propagating our genes. Because of the historically high overlap of "people you grew up with" and "people you share a lot of genes with", creating a biological aversion towards mating with those you grew up with serves as a good proxy for creating an aversion to incest (which is pretty much impossible to recognise without gene testing).

Their point is that it's not "taught" behaviour in the sense of being culturally dependent, like we are taught that "everyone should have equal rights", and "being naked in public is not ok". Both of those are cases where there is a massive variation between cultures and through history, indicating that they are things we are taught, rather than being biologically ingrained.

Are instincts not biological?

You're talking past each other: Chicken is saying that we have a biologically ingrained aversion to mating with those we lived closely with up to the age of 6, and is suggesting that this is a result of evolutionary pressure making it preferable to avoid having kids with your nearest relatives.

Their point is that this is not "taught" behaviour in the sense that it is imprinted from culture (such as "what hairstyle is attractive" or "what music is pleasant"), but appears to be biologically grounded.