I would think that in the early vaccination groups, the overlap isn't nearly has high. People who are 65+ or at very high risk have very likely been taking more precautions and therefore will have been infected at a lower rate over the past year.
Using Florida's data, there are 2,891,608 people in the 65-74 age group and 156,930 confirmed cases which calculates to 5,427 per 100k. In the 25-34 age group there are 2,839,285 and 327,208 documented cases which calculates to 11,524 per 100k. That doesn't even consider that the younger group are likely to have more asymptomatic, undiscovered infections.
There will be less than half of the overlap in the 65-74 age group as there will be in the 25-34 age group which further solidifies the philosophy that starting with 65+ was and is the right decision. More bang for the buck towards herd immunity as well as in eliminating the maximum potential fatalities.