He doesn't want to work on mold as part of Google or another large company like that because then Google would take ownership of the whole program; he wants to maintain ownership of it. It's mentioned in a tweet of his

Doesn't Google pay people to work on the Linux kernel, even though Google can't take ownership of the whole Linux kernel?

Google owns employee code outside of work if I understand things correctly.

So Google only has copyright over the portions their employees have contributed.