Can they all vote immediately?
Probably.
Who’s going to stop them?
The California DMV offices are staffed by SEIU members. Most of them wear purple SEIU shirts to work. You can be sure that all of those illegal aliens walked out the DMV office as registered voters. California voter registration cards have to fields for voter address. One is for where the voter lives. The other is for where the ballot is mailed to. The ballots will get mailed to an address controlled by the SEIU.
Ronald Reagan would have never been elected governor with such shenanigans.
Does the DMV vet citizenship claims on their form? I suspect the clerks would all think it is above their paygrade and they just pump the paper through however it is filled out.