There are bits that both parties copied from FreeBSD. It is perfectly legal for both of them to do this.
SCO Unix today is basically a re-badged copy of FreeBSD 10, as they ditched their legacy codebase.
Also, remember the three hurdles they have to overcome:
1. They don't own the copyright to Unix. Novel/SuSE owns it. Not sure which one now with the recent reorganisation, but it isn't SCO.
2. They published their own copy of Linux in the form of Caldera. Therefore they would have granted a licence to any copyright they might own under the terms of the GPL.
3. They need to prove that any code was written by them and not by for example FreeBSD.
Biting the hand that feeds IT
