The CloudFlare blog concludes with:

"By bringing celld and workerd together, we want it to be radically easy to build and operate distributed applications on your own infrastructure."

It does not appear they are close sourcing it. He mentions workerd is open source. Roadmap needs better communication but it appears people will be able to switch to new merged OSS version. It's just not well defined what it looks like yet.

We don't completely know what it will look like yet either, but it will absolutely be open source -- as workerd and celld are both already.