SCO was suing on two grounds. First was the claim that they were the corporate heirs of the AT&T copyrights, and they claimed that Linux infringed on them. Second was from a joint development agreement with IBM - they claimed that code from that had been contributed to Linux by IBM, and that IBM didn't have the right because it was half SCO's code (or rather, Novell's code that SCO inherited).

That second claim could not have happened until IBM was contributing to Linux. That part of the lawsuit could not have happened in 1993. (The first part was similar to the BSD lawsuit.)