If this was done to kill celld as a runtime that would be unfortunate.

edit: my reaction was too soon, it seems like they will be explicitly working on making workerd an open source self hostable runtime

On the contrary. Please read my part of Cloudflare's blog post here: https://blog.cloudflare.com/deno-joins-cloudflare/

Sounds like the opposite: They are killing Deno to work on celld