According to Phinney/Volek they didn't lack for sodium supplements. The Masai get it from drinking cattle blood and the Inuit got it from ice of a certain age. I think I quoted them earlier in the thread.

Salt is pretty easy to acquire, especially if you live close to an ocean, or a primordial ocean and our ancestors weren't idiots.

Also, they probably didn't all eat very low carb diets. The more carbs you have, the more your body hangs onto sodium.