This says that healthy diets are a sufficient factor for healthy teeth, but they may not be necessary. So early humans might have had the same diet as us, and something else could have protected their teeth. We don't know, so this does nothing.

Regardless of whether or not early humans had cavities, we already know that early humans had overall fewer dental problems than we do now. We need to strengthen the claim this is because of the difference in diet, so whether they had cavities is irrelevant.

We don’t know if varied diets necessarily make a difference to dental health. This isn't relevant because it does nothing to differentiate early humans from modern humans.

This weakens the author’s argument. Early humans had fewer dental problems because they didn’t live long enough to develop such problems, rather than because of their diets.

If two groups have different dental health, it's almost certainly because of their diets. This affirms the author’s assumption that early humans having fewer dental health problems could only have been due to a difference in diet.