Yes, it's allowed ('though the name is trademarked). However, forking is usually harmful (effort spread thin, divergent APIs, plugins, integration with other tools such as Maven, Gradle, ...).
On that very email chain, once the sitution was stated and as Bill kept on ignoring everyone as he did over the last year and a half, the community hard forked FindBugs and started aligning behind https://github.com/spotbugs/spotbugs
Bill's reappearance is now puzzling. To abandon the fork to go back to FindBugs would require major leadership changes, as confidence on Bill has been shattered. Keeping the fork leaves the project on the same point people tried to avoid in the first place.