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Actually he is both (read his CV), but the field he works in and he is most famous for is computer science. TAOCP is about computer science, TeX and MF are works of computer science. Of course, all of that requires some mathematical background, too, but that's actually typical of computer science. You can hardly do computer science without mathematics.Do you really think that I do not know Knuths "The Art of Computing Programming"? If you read it carefully, you note that Knuth is not a computer scientist, but a mathematician.
Note that many famous computer scientists are also mathematicians. Historically, the field of computer science was branched from mathematics (at the beginning, computer science was barely more than a variant of applied mathematics). When I started studying computer science, the first four semesters had the same lectures as my fellow students who studied mathematics.