Are you sure? The React compiler is a fairly new addition to React, Flow is Facebook's old alternative to Typescript, but Typescript won the ecosystem in terms of broad adoption in the end. I think Flow is barely used today, I would be really surprised if they choose it for a new tool, even for a Facebook project.

You may be thinking about React itself, not the new compiler? I'm sure there must be some flow in there still from back in the day.