Maybe I should have been more explicit that here "variance explained by" is used quite literally with the meaning it has in statistical modeling. I get that you meant it in a "correlation not causation" way, but "correlation" in the context of statistics is a loaded term. Interpretability of heritability is difficult in general. Most people struggle with it. But if it truly were 100% (rarely is if ever) it would imply causation solely from genetic factors. Causation from both genetics and environment is always there - heritability just seeks to formalize the measure of proportion.