The lawsuit would be of the form "you do not have a license to distribute the code under those terms". Since LibreOffice is obviously based on existing work, the primary defence would be to show that they do in fact have a license for what they're doing.
In such a case, it would be pretty obvious that LibreOffice was derivative of the LGPL'd OpenOffice.org code from Sun/Oracle. It would be more work to prove that their work is derived from the Apache licensed release.
In contrast, if they had version control history that shows how a new version of LibreOffice was derived from the Apache licensed release, then the defence would be a lot easier.
I don't really know the specifics of US law, but in my country civil cases like this are decided "on the balance of probabilities" rather than "beyond reasonable doubt", so not offering a defence could easily lead to the case being decided against you.