Yeah- probably not who you voted for - but that you voted would be a public record.
I am imagining that in this exercise - Cambridge Analytica knew that even a large presidential election turns on a relatively small number of voters in specific states and precincts. They could probably identify the specific states they wanted to target - PA, OH, WI - and find those precincts where they could flip/suppress enough votes to alter the outcome. But, even once you narrow that scope - its still a daunting task to figure out who to target specifically, and with what message. Voter rolls will have voter activity, and in most cases party registration.
I think this is helpful, to focus your campaigns on specific people, with specific voting histories, and with specific party registration in specific precincts, in specific states.