They actually save a good chunk of money by doing this instead of going the First round tender, then franchise tag route. I don't think you can give him a long term deal in 2019 with two (soon to be three) massive contracts on the offensive line already. So a franchise tag would've been in play.
2017 First Round Tender: 3.91 mil
2017 Franchise Tag for OL: 14.271 mil
Total: 2 years, 18.181 mil
This deal: 2 years, 15.4 mil
Savings: ~2.8 mil (but likely even more with the franchise tag for O-lineman increasing about 1 mil every year since 2013.)
Shrewd deal.