As far as this case goes, that is a very good point. However, Darl McBride has publicly claimed Linux was tainted as of "early 2000."
Darl's big mouth won't matter until we get to the countersuits. However, SCO's official position, as documented in paragraph 5 of its Second Amended Complaint is:
.. Linux 2.4.x and 2.6.x and the development kernel, 2.5.x, are replete with protected technology. As such, the Linux 2.4.x, Linux 2.5.x and 2.6.x kernels are unauthorized derivatives of UNIX System V.