It's interesting that from time to time there's a lot of complaining about managers not using their best reliever in the highest leverage situation. Then when it happens, people complain.
As someone notes above, Duran didn't face 8-9-1, he faced 9-1-2. That meant Duran (our best reliever, right) faced a righty, a lefty who is also the only guy in the lineup with an OPS over .725 (Rojas at .979) and the righty who's seen as far and away their best hitter (Rodriguez). That's not a good split mix for Thielbar, but Duran is probably the person on the team best equipped to face that. If one of them gets on, the next guy up is the switch-hitting Polanco, where there's not a platoon advantage either way.
Assuming Duran gets through the inning unscathed (and that's the hope/plan, right?), it means Thielbar gets two switch-hitters and a righty. By definition, the Mariners have at least two platoon advantages in their favor either way. They are also OPSing .638, .626 and .725, with batting averages of .195, .214 and .206. That's much softer than what Duran faced. And if, as @2wins87 says, the switch-hitters are weaker from the right side, they ought to be facing a lefty.
There's also another aspect. The Twins batted in between the two innings. Seattle has a good bullpen, but if the Twins score in the bottom of the eighth, it changes the top of the ninth. Said another way, Duran was guaranteed to pitch the situation where the Twins are up one. Assuming Duran does his job, Thielbar could have entered with a one-run lead, but he could have also easily entered with a two-, three- or even more-run lead, which lessens the leverage even more. Ignoring the inning/handedness, would you rather have Duran pitch the inning with a one-run lead and Thielbar with the one, but possibly more run lead or vice versa. I'd rather the former.
In my mind, Rocco played it correctly.