Law of large numbers. The widespread plagues of the past were never 100% lethal, or we wouldn't be here - a sizable fraction of the population survived, and over several rounds of infection/reinfection gained something we'd think of sterilising immunity.
In the meanwhile, the plagues and their knock-on effects may have eliminated approximately HALF of the entire population of their times.[0]
By letting people die. If covid came around in the 1200s, then everyone would have gotten it and ~3% of the world population would have died. Definitely suboptimal but nowhere near an extinction event.
Given COVID’s risk/age relationship and already lower life spans in that time period plus higher vitamin D exposure rates it’s quite likely rate of death would have been lower in that population.
The question was about a world without modern medicine. It's totally reasonable to speculate there would be a higher fatality rate without medical intervention keeping people from dying.
In the meanwhile, the plagues and their knock-on effects may have eliminated approximately HALF of the entire population of their times.[0]
0: https://en.wikipedia.org/wiki/Black_Death ("is estimated to have killed 30 percent to 60 percent of the European population")