I know I have read that when they leave - they have had this planned for a long time and their "grieving" took place long before - when we had no idea it was going on. So now that the leaving has occurred, we are now going through our turn. This explanation disturbs me so much - not that I doubt it is true - but just in terms of the unravelling it does of the time we were together.
I am honestly not sure that I believe this, at least for my own case. I think my xh was shocked at what he had done, and a lot of his subsequent crisis has been dealing with the shame from having treated us all so badly.
I absolutely do not believe he had checked out a long time previously. Not do any of the people who knew him well, including his three children.
Ultimately we have to trust ourselves. And I tried to 'believe' that the statement you quoted was true, but I know it isn't, for me and my marriage at any rate.