I suspect that when you buy decaf beans from a roaster they don't aren't the ones decaffeinating the coffee beans. I think roasters have a lot less choice when buying decaffeinated beans so I bet your theory is correct.
The Mexican one is supposedly superior in taste, but the majority is processed through Vancouver with the Swiss Water plant. https://www.swisswater.com/blogs/sw/swiss-waters-new-decaffe...
So, the "roaster" (if they don't use a separate company for that) does have the ability to select the exact same beans between their decaf and regular coffee products. Whether they sort the beans they get from the farmers and choose which go to be decaffeinated according to some criteria, is a separate thing.