You are correct - John Irving writes his endings first.

https://www.backwordsblog.com/single-post/2016/04/27/john-ir...

"People have commented – sometimes with irritation, sometimes nicely – on the amount of foreshadow that there is in my novel. Well, yeah, it’s not that hard to foreshadow what’s coming when you know what’s coming…I write endings first. I write last sentences – sometimes last paragraphs – first. I know where I am going. I write collision course stories."