That's quite a meaningless statement. Molecular genetics is in its infancy and so you're appealing to absence lack of evidence, which exists for most of everything, as evidence of absence - quite nihilistic! A quick search similarly suggests rates as low as 10% for the heritability of (non-European) height based on molecular genetics, undoubtedly for the same reasons.

And we've already had this conversation and in spite of your obsession with bias confirming 'blogs written by very smart people' - the heritability of TV watching is zero. Actually lol looking this up, it turns out there has indeed been a study done on this [1], as part of a study on the heritability of sedentary behaviors in general. And yeah, it's zero.

You are right that the number of hands you have is not heritable, but is only because heritability is defined as the percent of variation in a trait, between populations, that can be explained by genetic factors. When basically everybody has two hands, there is no variation, so there is no heritability to measure. It'd be like discussing the heritability of hair color if everybody had black hair. It's not some sort of clever insight or retort, though I'm certain your blogs framed it as the wittiest thing ever.

[1] - https://pmc.ncbi.nlm.nih.gov/articles/PMC4193845

This comment doesn't actually say anything. I get that you're unhappy with the comment that I wrote? But that's all I get from it.

The one specific claim you make here, that height has heritabilities measured at around 10%, is false. In fact, the comparisons between the steadfastly high heritabilities of simple physical phenotypical traits like height and complex behavioral characteristics is a big part of the current scientific conversation about behavioral genetics.