This isn't actually true. Not all GNU projects have copyright assigned (although many of them do).
The point is that GNU is trying to build a coherent system. If it takes a project it will sometimes continue with it, even if the people who originally developed decide that they no longer want it to be part of GNU. In this case, as RMS says, this was not the right route because libreboot had not been part of GNU for long (so removing it causes no issues), nor did they have anyone who wanted to maintain it as part of the GNU policy.
If you think that this is weird, it is one of the cornerstone freedoms. Likewise, you could take any GNU project, and create a competitor with a different name today, using their code. It's happened before.