Look at prisons with bad nutrition and you’ll notice a large amount of people with some serious muscle mass. If you’re getting regular, very intense exercise (as you would need to as a hunter-gatherer) you’re in the top x% of the world population by fitness, no matter if you’re slightly nutrient deficient. Furthermore, we have at least some reason to believe that prehistoric populations weren’t so badly fed that it led to physical issues - they regularly reached heights equaling that of humans in recent years, far taller than the poorly fed people in e.x. medieval Europe.

Furthermore, modern humans have plenty of disease and issues caused by our modern environment that would render us far weaker 70 kya. Near/farsightedness, teeth issues, diabetes, obesity, repetitive stress injuries, etc. are entirely modern issues. Then look at the comforts we’ve gotten used to (shoes, clothing, soap, a too-clean environment for our immune system etc) - and I have no doubt that a modern human would be in far, far worse shape for the prehistoric life than your average person 70 kya.

Prisons with low calories nutrition have weak thin people in them. Muscles don't grow of you don't eat protein and sacharids - no matter how much you exercise.

You can have sucky nutrition for lack of vitamins etc and those will eventually leave you in bad physical condition. Even if you gain muscle mass temporary, damage to health will be real.

Hunter gathrer also don't have reason to bulk up unlike those prison guys.