I don't really eat any prepackaged foods besides macadamia/coconut milk. Mainly broccoli, spinach, celery, eggs, chicken, turkey, tuna and a ton of coconut/olive oil. A tablespoon of salt is 7080 mg of sodium. There haven't been any studies looking at ideal sodium intake for people on keto as far as I know, though it is generally accepted that there is a higher need since the liver gets efficient at excreting sodium.
I wouldn't necessarily recommend others take as much sodium as I do, only that personally I seem to feel better around that level. I have heard from some people that take as much as 11g a day on keto. I would definitely not exceed 5g much if I was on doing a standard diet though.
EDIT: one tablespoon is 18 gm of salt. That's over 6 gm of sodium.
EDIT2: Also, the study you're talking about says this:
> We noted that most of the world’s population (about 95%) studied consumes more than 3 g/day of sodium, regardless of hypertension status and only 22% consume 6 g/day or more of sodium—the threshold above which we note an increase in mortality and cardiovascular disease risk.