To me, the thought process was probably "we could get Espinoza back, but there's a chance the Pomeranz can pitch for the rest of the year during this stretch run. It's too late to trade for another starter now and we really need all the help we can get with this rotation."
If they were offered this remedy prior to the deadline, I bet they would have reversed the trade.
We'll never really know though. It will be oh so enjoyable to talk about this for the next decade or so.