In Texas, everyone redshirts their kids in pre-K 4 to start kinder when they are a year older than they should be. It is done to give their kids a boost in athletics and college scholarships.
While I have no specific info on Cook, but her parents were Div I runners at TAMU. Her mom ran the US trials for 5k and 10k on the track. Her dad is her coach.
Cook's HS, Flower Mound, which is in the north Dallas burbs, is huge. The school has over 3,500 students. Parents who have kids in these huge districts know that there kid is going to have to be a near national quality athlete to be able to participate in varsity sports. And it is even worse in sports crazy Texas where parents spend five figures on elite teams, coaching and training to get kids spots on baseball, volleyball, etc. So, a red shirt for kindergarten is almost a pre-requisite if you want your kid to be in HS sports.