I can think of three possibilities about the ex's alibi for him (besides that it's true).
Maybe she knew something and failed to report it. Maybe she is legally culpable for not doing something when she had an obligation to, and she doesn't want him convicted of something she could be ensnared in.
Maybe she doesn't want the father of her 3 sons to wind up a convicted child murderer and be executed.
Maybe she has confused a date or something in her memory. How do we know she doesn't have a poor memory or maybe mental difficulties and is confused.
I don't know.
Have to wait and see.
It is entirely possible, though, that this whole confession thing is an act and a crock.
Hardly unlikely, since she actually married this guy.