First noting that nothing about this is substantiated or even educated. Just something I've had in my mind for awhile and not exactly a hot take.
I would bet money that Joe C knew he was retiring in 2025/2026 by at least 2021. At the time, he tells Zac to go get an AD job, which he would get him with one phone call, and the perfect one opens up in Mississippi St where it's in the SEC, does not have the pressure of the big boys, and allows him to get the job title. The timing of Joe's retirement was also to make sure that Zac's tenure (wherever he went) was not long enough that people could point to results (good or bad), but long enough that when he's announced as OU's AD nobody can point to a lack of experience.
I'm also not stating my opinion on whether this was/is the right or wrong move/hire for OU, but all seems very plausible with a controlled and measured guy like Joe C who wants his legacy to continue in hands of his choosing.