Wait. You want more don't you?
Shawn Kelley is the best, most proven reliever the Nats have. His K/9 is fantastic (over 11.5 over the past 4 seasons). His BB/9 has moved steadily down from a "uh-oh one of these guys" 3.88 to 3.48 to 2.63 to a "did somebody say elite?" 1.71. His HR/FB is perfectly reasonable. His percentage of hard hit balls is not high. His LD% is low.
Blake Treinen is a GB specialist. Over 65% last year and never under 59% in the majors. Along with the groundball inducing good sinker comes a lot of mediocre contact (Hard% on the low side). However, he doesn't strike out nearly as many (K/9 around 8.5 over past 2 years), and walks way too many (over 4.00). Plus his HR/FB is higher (though not terrible). With not too much bad luck a Treinen outing can go horribly wrong.
Koda Glover is an unknown. In the minors he has great stuff (K/9 of 10.6 in upper minors) and very good control for his age and that stuff (2.2 BB/9). He seemed to be unhittable as well, having a hits per 9 of under 7 across every level since being in the Nats organization*. There's also less than 100IP total for Koda and he hasn't pitched more than 24 innings at any level. That means there has never been time for the level to adjust to him. He also seemed to have issues pitching with men on - showing a horrible LOB% and allowing 2 of 6 inherited runners to score. Small sample size? Almost certainly. However it highlights that Glover is an unknown more than anything.
In short, guys don't hit Kelley well, they don't get on base against him, and if he needs to he can strike them out. Guys hit Treinen terribly, but they can get on base against him, and he can't necessarily dial up the big strikeout. Guys hit Glover worst of all, they don't usually get on base against him, and he can strike them out as well, but these "guys" are almost all minor leaguers.
So Shawn Kelly is objectively most likely to give you the best major league results. By the whole "closers are stupid" philosophy, shouldn't he NOT be pitching in the ninth? Sure. If this were a perfect world. I'll give you a moment to check on that.
Not perfect, huh? You see if Shawn Kelley isn't the closer then he'll be tied to some other role, likely 8th inning guy so what exactly does it matter where he is in this stupid one-set inning food chain? If Dusty were going to break with tradition and use him all over the place then maybe I'd say don't use him as closer but I don't have faith in that. Therefore - might as well give him the inning that's most likely to find itself with high leverage situations.
Another benefit of Kelley the closer - limited innings. Kelley doesn't have any flaws when he can get the ball from the mound to the catcher, but his career is already filled with injuries and last year ended with a dead arm in a playoff game. Closers, because they are used very specifically, pitch fewer innings than even the 7th/8th inning types who are more frequently called on to get an extra out or pitch in closer or tied games. It's possibly smart to limit his innings and sticking him in this role may serve that purpose, if secondarily.
*Kelley and Treinen were both this low last year, too. But historically have been higher so there's more a chance of them being a little easier to hit.