Even then you should be careful with marketing. My favorite Mac clone of Tetris called Quinn (
Simon Härtel — Quinn) actually got in a little heat over it. His site said something like "a popular clone of Tetris". He got to keep the game, but they made him call it a "tetromino game" instead of a "Tetris clone" and he also had to put this on his page: "Neither Quinn nor anything on this site is affiliated with or sponsored by The Tetris Company or part of their Tetris line of products."
So yeah, Sir Tet itself should be fine as long as he never put "Tetris" anywhere in the game or website. Even using the name promotionally can come back to bite you in the butt as shown in the Quinn situation.