This isn't universally true. There were less incidences of general tooth decay due to different microflora than we have now, but people absolutely still got dental issues that would result in systemic infections and death.
I used to work in a fish market that processed wild caught fish fresh every day. Nearly every single fish we cut had some worms in it. Cod was the worst, salmon was middle of the road, flat fish like flounder seemed to be the least wormy but still had it. Maybe they were just harder to spot idk.
The original claim was dental issues were generally a death sentence, which was true whether or not they're more common now.