Ferrer didn't really come out of nowhere. He was just hurt. His minor league numbers were good all the way up to AAA and he played in the Futures game. There's just never a ton of prospect buzz for pure relievers.
He's been in the Nats system since 2017 and is 24. He was mostly okay in the minors but not good last year when he came up with the Nats. Much better this year.
Regardless, by all accounts the Nats have improved their development system. Irvin has given the development system credit for some of the improved young pitching.