I agree that Sheba has tabby markings in her dark patches. There are lots of ways to describe this complex and beautiful color and pattern. Caliby is one term you could use. You could also use the terms brown patched tabby and white, or torbie and white.
A calico would be a genetically solid cat with black patches, red tabby patches, and white areas. Sheba is genetically a tabby cat with brown tabby patches, red tabby patches, and white areas. So I agree with you that Sheba isn't technically a calico, however some people informally use the term "calico" for patched tabby and white cats like Sheba, too.