I think it's more likely than not that Jeffers is retained at least through Opening Day. The move is just too low-wattage to truly be driving decisions in any direction. Just like attaching Dobnak's buyout to the Paddack trade (the only purpose of that was to pass the burden of his buyout off onto Detroit) wasn't proof that they're cutting costs as much as possible. They are not meaningfully worse off if they do ultimately decide to sell off anything of value in a true rebuild. If anything, it's a treading water move that doesn't preclude them from going in whatever direction ownership dictates with their budget.
I also wonder if catcher might be a unique position in that it can aid in the development of others on the team (young pitchers in this case), which could potentially add value even if you're not caring about wins and losses. Most positions are kinda doing their own thing for the most part, but if they think Jackson is in a better position to do that then Rortvedt, Pareda, or anyone else available in the dumpster, then I can see that as plausible. I hadn't heard of Jackson before today so I have no idea if this is true, or if this is another miscalculation in player evaluation, but I think it could make some sense. I don't know if I'd feel the same way if they'd made the same kind of move for a 1B, for instance