Billboard.biz:

On Thursday, West and Jay-Z indicated how they intend to fight the lawsuit, denying much of the allegations except with the acknowledgment that their song, "The Joy," which allegedly included Johnson's work, was released prior to "Watch the Throne" and was also included on a deluxe version of the album.

Their lawyers maintain, essentially, that the recording was made before 1972, and therefore "not protectable," when the law was changed to include sound recordings (PDF):

Sound recordings fixed before February 15, 1972, were generally protected by common law or in some cases by statutes enacted in certain states but were not protected by federal copyright law. In 1971 Congress amended the copyright code to provide copyright protection for sound recordings fixed and first published with the statutory copyright notice on or after February 15, 1972. The 1976 Copyright Act, effective January 1, 1978, provides federal copyright protection for unpublished and published sound recordings fixed on or after February 15, 1972. Any rights or remedies under state law for sound recordings fixed before February 15, 1972, are not annulled or limited by the 1976 Copyright Act until February 15, 2047.

h/t: Prefix