Most of human evolution occurred below 40N, so even if that point were correct, it would at best be limited to the adaptive behaviors of a very small group of human ancestors.
However, there's plenty of evidence suggesting that there wasn't any such regionally adaptive behavior. The original find that suggested starchy plant consumption in H. erectus was analysis of the Dmanisi fossils at ~41N. Later work tying these sorts of results to typical European Neanderthal diets has been done for North Sea sites (~50N). Of course, Neanderthals never went much farther north than that due to the climate. As for AMHs, I've seen papers of sites near Smolensk (~55N) indicating moderate to heavy starchy plant use. That's not far from the glaciation line.
Moreover, there are fairly convincing arguments that a nearly pure-meat diet doesn't work in winter due to the general lack of fat (e.g. https://doi.org/10.1016/j.quascirev.2009.11.016 ). There are caveats and workarounds here, but this whole idea that high-latitude hominins had limited, homogeneous diets is on increasingly shaky ground.
