He was probably made to fall on his sword.
The resignation was probably forced in order to shut him up.
As a private citizen he can be called but cannot be compelled to testify in detail without clearing everything with the CIA first. Which they will stonewall of course.
And the real blackmail is this investigation, not the affair. They still have a hold on him (and her).
If true, that as a private citizen he has to clear testimony with the CIA first, then he should have resigned after his testimony.