The only reason number of arms is not heritable is because the variance is close to zero.
Once you include a sample with non-trivial variance (a sample including humans, snakes and spiders), you discover that number of arms is almost 100% heritable.
You're missing the point. Saying something is "heritable" is essentially meaningless. It tells you how that trait would change in the group measured in response to evolutionary selective pressure, and that's it. It doesn't tell you whether the trait is genetic, it doesn't tell you whether the trait is important, and it doesn't tell you whether the trait even exists in a meaningful way.
You can measure quite decent heritability from entirely random traits:
Once you include a sample with non-trivial variance (a sample including humans, snakes and spiders), you discover that number of arms is almost 100% heritable.