AAA has long been a parking lot for borderline guys and not a developmental level. I know not every parent club views it like this but most do. A great deal of the guys do skip from AA to MLB or just use AAA as a speed bump if there is an injury on the AAA roster.
This is solely about service time manipulation so they can get an extra year of talented guys before becoming free agents. Doing such is illegal in the CBA but it's almost impossible to prove and the players union has long cared more about current players than future and have more things to worry about to start protesting these actions. Especially, if the kid gets called up before the dispute hearing is scheduled. The last CBA that gives clubs incentives to have rookies on opening day roster - compensatory picks- only applies if that player finishes in top-3 of rookie of the year voting. The odds of being top-3 are just too low.
Trey will be called up shortly after that magical 16th day of the 187 day season.
The pitching is normally better at AA than AAA. If the Nats were truly wanting to develop Trey, they would have sent him back to AA. Sending him to AAA just looks better PR wise.