The argument concerns only the human trait of disliking vegetables. The author doesn’t make any assumptions about all other human traits.

The argument doesn’t generalize from the members of the study to the human population generally. The conclusion is just that a dislike of vegetables is sometimes genetically determined; those times could involve just members of the study.

The conclusion does not assert that a dislike of vegetables is only caused by genetics. The conclusion is merely asserting that a dislike of vegetables is sometimes caused by genetics. This acknowledges that such dislike can also be caused by other things besides genes.

This possibility doesn’t undermine the argument. If dislike of vegetables can also be caused by other genes, that supports the conclusion that a dislike of vegetables is at least sometimes genetically determined.

This must be assumed, because if it wasn’t true, the conclusion wouldn’t follow from the premises. If the group that liked vegetables also all had the XRV2G gene, then there’s no reason to think from the study that the XRV2G gene plays any role in dislike of vegetables.