It's practically nobody's position that there's no linkage between genetics and intelligence (that would be weird indeed), but it's important for this comment for you to believe that's the counterargument --- otherwise the comment doesn't make sense.
If genetics is less than 5 percent I would consider that something worth ignoring.
If it is 10 percent it is substantial enough to make a difference at the extremes.
If it is 20 percent that is real serious business.
Anything higher means we should really sit up and take notice of this fact.
If you're arguing against a eugenicist then it's not just about the percentage, in that case you have to distinguish between genetic and heritable. Suppose that there are some set of four genes that, in just the right combination, are worth 5 IQ points. That's, by definition, genetic, but it won't have a strong correlation with heritability because every kid has four chances to get the combination wrong. Or, if the combination does something bad, four chances to get it right even if their parents didn't. So past performance is no guarantee of future results.
By contrast, if you're trying to decide whether to allocate more resources to kids who already show promise, you care about the individual's natural potential rather than the statistical probability that it will be similar to their parents, so it doesn't matter what was more likely, it matters what actually happened. And by the point you're performing the evaluation, you can't go back in time and change things like the prenatal environment for someone who is already born, so in that context those things belong in the "nature" column and "nurture" only gets the things you could still affect.
Let the science science, and policy makers make policy.
I think the problem comes when we want scientists to make policy recommendations.
I think scientists should help us determine what the facts are, not decide what to do about them.
What to do is for courts and democracy and for individuals to decide?
The evidence is overwhelming in that direction:
Twin & family designs: h² ~0.4–0.8, often ~0.7–0.8 in adult samples like MISTRA.
Adoption: people resemble their biological relatives more than their adoptive relatives in IQ, despite strong environmental differences.
Molecular genetics: polygenic scores and GCTA show real, replicable genetic signal for intelligence.
What it does not mean is that:
1. IQ is fixed at birth in a way that cannot be influenced by environment.
2. IQ differences justify any sort of discrimination or moral ranking.
Those two points are where people tend to clutch their pearls and panic. Just because we have noticed an uncomfortable truth does not mean that it is valid to use it in a discriminatory manner; the issue becomes that people will inevitably do so, thus those "in power" cripple the theory in the crib so as to avoid the fearful and uncomfortable implications.