At the time we had Zay, Matthews, and Boldin with a healthy streater who was actually looking like a solid roster contributor. It wasn't a great situation, but they wanted the 2018 2nd round pick more than the POSSIBLE 3rd round comp pick in 2019 - a full year later and probably 30 picks later. That's a significant difference.
We also got gaines in the trade, who's actually been pretty productive so far.