There is also the issue of Dawkin's pleas. Justice and FBI may have been OK with him spilling just enough beans to hang himself, without getting into other arenas of what each player and "circle of associates or family" knew, and agreed to in principle if not in precise arrangements, or what they didn't report. Those reporting mandates aren't criminal violations, but might be NCAA violations under their vague "interested parties influence" rules...
FBI / Justice agreed to give NCAA a bunch of amassed evidentiary materials, of which that damning email was a part of, but didn't necessarily have any obligation or interest in helping NCAA do it's ethics job beyond that.