If the Sox are very confident that Thompson will never amount to anything, the time to trade him is right now. However, you aren’t going to get a young prospect for him straight up. So you make this trade, hope Horn pitches well enough, and then you flip him for a young prospect. It would basically be flipping Thompson for said prospect, but the only way to get there is this indirect route.
I have no idea if that’s the actual plan here, but that’s what I’m going to tell myself to feel better about the trade.