I think DD got snookered by Boras, to be honest. He just loves Castellanos, and thought he'd be a clubhouse chemistry-changer as well as a big bat (which he well might be, but they still didn't need this many DHs). And the market became team-favorable, when there wasn't much else left (like pitching. SS, yes, but clearly they weren't willing to add to that logjam).
Maybe he'll be able to change the mix on the trade market, and it's not like they won't have money to spend with Didi and Segura gone. Bohm, Stott and an early arrival in CF would be big. But there's also no real rotation help on the way with Gibson and Eflin gone this year and Nola next year. Presumably if they are in it they will try to get a controllable arm this deadline, as they did last year.
But being on this path also pretty much guarantees they will trade a lot of prospects over the next two years (that's all Dombrowski has left at the moment) if they are actually in it.