It wasn't worth $4.6M for one year to see if he could be a useful piece in a bullpen that needs a couple more useful pieces?
This site has them at just under 181 million before arbitration.
They have:
Arb 1 - Bogaerts, Holt, Leon, JBJ, Workman and Thornburg
Arb 2 - Kelly, Pomeranz and Ross
Arb 3 - Abad
Ben Buchanan at OTM estimated they'd spend around $25 million in arbitration this year, so that puts them at around $206 million or so. Even if you think that arb estimate is high, we're still not incorporating the other factors that go into payroll like medical benefits which usually run somewhere around 10 million, so you could cut his number in half and they'd still be at a little more than $203M. If his guess is accurate, they're way over the threshold.
So no, they didn't have 4.6 million to roll the dice on Koji not being toast. At least, not yet they don't. Plus, I'm not sure I'd want him on the team ahead of any of the non-optionable bullpen arms they already have to try and squeeze onto the roster, except for Abad who probably isn't worth going through the Arb process with. Koji was phenomenal in 2013 and 2014, but he's oft injured and fairly pedestrian these days, mostly because he is so homerun prone.
It's tough to be a dominant reliever with an 87 mph fastball. Tip your cap and move on.