The way I understood it was that when we train, over months/years the body will create new blood vessels as needed to support the increased workloads. These don't go away when an athlete lowers the workload during retirement from training. They are still there. So, when the athlete takes up sport again...any sport...they just recruit the pathways already in place.
There are countless stories in various sports like this. I'm vaguely reminded of a female road racer or TT specialist that retired to start a family. When the children got to a certain age she started riding again and was making the podium at world class events within months. I wish I could recall her name. She's either American or Canadian.