I have noticed this in general.Certainly this is my opinion not supported by hard research. The more fit someone is the more likely they are looser and more flexible. The idea of " move it, or loose it" I believe is true and positively correlates with what I have seen from coaching kids and adults.
The fitter you are the fitter you can be. Those who have, get more ( and therefor are able to get more. ( two of Charlie’s fav sayings about playing to your strengths and doing what can be done opposed to complaining about how it can’t be done. I keep this in mind constantly especially when training circumstances go off kilter. This is a general principle I learned a long time ago and it has served me well both personally and professionally.
Very fit athletes might temporarily lack flexibility depending on the training phase thru the year but I think once an athlete prepares to compete adaptation has occurred and range of motion is restored. ( I also think there is a wide range of people in this regard and likely fitter and more flexible people end up in sport due to some of these born attributes )