It's really only the Orioles & KC; and Britton, Bundy and Hochevar all got long looks as starters first.
The Yankees developed Betances (as a starter, but that doesn't matter), Shreve was a Brave, Yates a Tampa, Miller, Chapman elsewhere.
Orioles: Britton, Bundy and Givens are home grown, Brach was a Padre
Royals: Davis was in TB, Herrera, Hochevar and Duffy are home grown (altho query the value of a 1-1 on Hochevar)
The Mets only home grown reliever of note is Familia.
Sox have Tazawa and Barnes.
Neither Smith nor Elias were the sort of "established" reliever that teams typically (and often wrongly, I agree with you there) trade for.
And certainly through today, Seattle "won" that trade. But that's a low bar for Miley -- 3 really shitty starts, 2 not so hot, 1 great, and 2 other very good ones.