By doing studies with identical and non identical twins, (looking at the differences between them) it is possible to have a vague idea how much genes influence things. There is a broad spectrum - some things are 100% genes (Cystic Fibrosis, Huntingdon's disease) some things are wholly environmental (language, religion, culture) and almost everything else is somewhere in the middle (Cancer? Obesity? Height? Personality?). The main idea is not establishing whether genes have an effect, but how much of an effect they tend to have.
The problem with saying that things like "language, religion and culture" are wholly environmental is that even those things depend on human-specific mental mechanisms, which are functions of a brain whose structure and function depends on genes. That's why a human child raised in a human family acquires the language of those around it, but a young chimpanzee does not.
Similarly, even genetic diseases cannot be said to be 100% "genes" (as opposed to environment) because the expressions of many genes are very dependent on conditions outside the genome, not least proper nutrition, exposure to clean air and water and sunlight, etc. This is a somewhat less fallacious claim than saying learned behaviors are "purely environmental," though.
But the point is, there's no real spectrum of genes vs. environment; everything about people (and every other organism) is a complex web of interactions between the two, of which we only understand the basics.
Most of Muz's assertions are (albeit sometimes oversimplified) standards of sociobiology and evolutionary psychology, fields which get attacked all the time on (ridiculous) ethical grounds but only occasionally on valid scientific ones...but that's another story.