If guests are using single day tickets, an adr might be the only thing allowing them park access during a phased closure.
On site guests and AP's are allowed in up until a full phase 4 closure. That's when no one else is allowed in. For people with ADR's that are not on site or AP's, an adr will get you in still during at least a phase 3, possibly even a phase 4. I can't remember if it extends to phase 4 but I want to say it goes until 3.
Wdw had only reached a phase 4 a few times since being open, so it's unlikely it gets that packed. So you don't have much to worry about with an ADR during busy times