I had been thinking that Ruiz still had an option that would enable the Nats to send him down to AAA and recall Millas, but I'm mistaken. It looks like the Dodgers added him the 40 man roster at the end of 2018 to avoid Rule 5, and used his first option in 2019. They also optioned him out in 2020 and 2021, so all options were burned by 2021.
I think he still has not recovered from his loss of strength with the flu, but he can't go back on rehab without a legit IL reason. This is a guy with who always has among the best contact skills and K rate in the game, and his K rate is now spiked from 10% last year to 15%. His swinging strike rate and called strike rate are about the same, and he's actually increased his zone contact, but he's just not punishing anything hard this year. Pitchers aren't beating him out of the zone as much as they are challenging him hard (fewer 4 seam but more sinkers, splits and cutters).
https://www.fangraphs.com/players/keibert-ruiz/19610/stats?position=C