I would guess that it has to do with the usefulness of the project outside of Google. This project could be applied to so many other things (as OSS-Fuzz demonst

I would guess that it has to do with the usefulness of the project outside of Google. This project could be applied to so many other things (as OSS-Fuzz demonstrates), so open-sourcing it makes perfect sense. It isn’t some kind of classified algorithm, either.