This sounds to me like it's quashing convictions for marijuana-related activity which is no longer a crime, and where sentences have already been served. So the convicts are no longer in prison, but they have a criminal record, which this project will wipe clean.
In a post from last year [1], a lawyer writes:
But here in California, there’s hardly anybody in prison for marijuana anymore because of the reforms in our system that have happened over the last decade.
[1] https://melmagazine.com/en-us/story/if-cannabis-becomes-lega...
There are more people who would be in if someone cared enough to raise key exonerating circumstances, judicial practices change since their condemnation. It’s less “straightforward” but it’s a lot less controversial than you would think.
This is why people who can read law books are so valuable in prison.