They buy a company, not a repo.

They buy the team, the know-how, the customers, the brand. The repo is open source up until now, but they can create a paid offering with premium features over the open source code. They can also offer specialized hosting for that solution (which is a very clear advantage of joining with cloud flare)

They might be able to relicense the codebase for people who don’t want (for example) to comply with the GPL. Although that doesn’t seem to be Cloudflare’s motivation for this acquisition.