It launched in early August 2021 (Shlegeris 2021).
I think that was referring to the research project, not the org itself.
Oh, I was assuming this was their first project, but on reflection the assumption was unwarranted. This other post, from August 2021, describes Redwood Research as a "new... organization", but I wasn't able to find their launch date. I've edited the article to address the issue.