Juvenile courts (in CA) only have exclusive jurisdiction over crimes committed while a defendant was under 18 (except for those enumerated crimes for which juvies can be tried as adults), and maintain that jurisdiction for all proceedings related to that crime until the defendant turns 21, at which point the juvenile court proceedings terminate by law.
Adult courts have jurisdiction over defendants for crimes committed as an adult (generally 18, though as young as 16 for specific crimes).
The arrest warrant was issued when Mitnick was 21 and on parole, or in other words, for a violation committed as an adult. While the arrest warrant was issued by the juvie court, it could have been issued by the adult court, as both courts would have had jurisdiction over the crime for which the arrest warranted was issued.
So the lawyer was right, because the statute of limitations for the adult court was tolled while Mitnick fled the jurisdiction. And the parole officer was also right, because the juvie court which issued the warrant would have lost jurisdiction over Mitnick once he turned 21.
Juvie court is way nicer than adult court. Mitnick "hacked" the system and only got away with it because prosecutors decided to focus their efforts on more serious crimes. But he could easily have ended up in prison for a few years.