It's probably worth noting that while Seattle probably didn't expect Miller to move that quickly, the organization has a very good reputation for pitching development. Miller, Bryan Woo, Logan Gilbert, George Kirby, and Emerson Hancock were all drafted in 2018 or later and have all made it to the majors; aside from Luis Castillo, their rotation is entirely homegrown. (And part of the package to get Castillo was Brandon Williamson, a 2019 second-round pick who's been a useful part of Cincinnati's rotation.) Compare that to the mixed results the Red Sox have gotten from their own homegrown group of Bello, Houck, Crawford, et cetera. Pitcher development is hard, but it's not magic, and it simply wasn't a strong suit for this FO.
As the article notes, the development coincides with the hiring of Max Weiner, whose job is (according to the article) as follows:
Weiner serves as a bridge in the Mariners’ player-development program. He works with every aspect of the organization — from Jerry Dipoto and Justin Hollander in the front office, to the scouts who identify pitchers with potential, to the analytics department that identifies interesting pitch qualities, to pitching coaches at various affiliates, to Woodworth and Trent Blank on the major-league staff — and pulls all the information together and tries to distill it down into something digestible for each pitcher.
Weiner’s gift is his ability to connect the dots, to bond with pitchers on a personal level, build a plan, build trust and build belief in that individual.
“He really understands his pitchers from a personality standpoint and from a competitive standpoint,” said Mark Lummus, a Mariners scout since 1999. “He’s so good at creating relationships with these pitchers, and they’re able to be the best version of themselves because of the time and investment he puts in.”
A little-used pitcher at Florida International University, Weiner spent much of his time in college learning everything he could about the art and science of pitching. He absorbed books on coaching, obsessed over pitchers’ biomechanics and asked endless questions of coaches and teammates.
That inspired him to start the Arm Farm, his program to develop young pitchers. That caught the attention of the Cleveland Guardians, who hired him to be their minor-league pitching coordinator in 2017. A year later, the Mariners lured him away.
Causation? Correlation? Magic? Luck? Skill? Would be interesting to find out. I'll note that the
old adage in baseball was that if a team has "four starting pitchers who are good in the minor leagues, is if [the team ends] up with one good starter and one good reliever out of those four, you’re doing pretty well". By internally developing four starters, they are an unbelievable roll. And if they can keep doing this, they're going to be tough out for years.