I don't think they were deregulated to work motors right away either. I remember them running on the Wassaic branch for quite awhile after, and the other F-Units worked the CT branches before the Brookvilles came fully online. The Amtrak P40's that eventually ended up going to SLE were the interim replacements for the F-Units before the Brookvilles showed up.
As far-fetched as Amtrak's plan seems for the Summer, there's plenty of well-regarded railroaders who have posted in this thread that I'm inclined to believe this is actually going to be a thing. The Amtrak employees who work the Hudson Division have never come off as distrustful or misleading to me in my travels out of Rhinecliff, I have no reason to disbelieve them now. I think this is a situation of trying to see the forest through the trees. It's easy for us to immediately shoot down the idea, because yes, there are several legitimate reasons why this plan couldn't/shouldn't work. However, people experienced at Amtrak, who have skills in doing what they do, and do it for a career, must see something that the rest of us posting on a railfan forum do not. Personally I'm happy that Amtrak's looking into various options instead of just going "¯\_(ツ)_/¯ sorry, Poughkeepsie's only ninety minutes from Albany, just take Metro-North from there".