I don't think the official story on the Mega Drive version of Tetris is known.
Since there's an estimate of not more than maybe 10 copies in the world, we can only speculate. It is likely to have not even been officially released. Perhaps made but release withheld. It is extremely likely the Nintendo lawsuit made Sega take pre-emptive action, as I thought I read a blog post a few years ago where Sega did confirm they did not have the license to release it on the home Mega Drive (though it was eventually released one of the Sega Ages for PS2, I think). I played that version a bit and it's pretty average anyways. Maybe nice to play it for the history.
The game doesn't have an official release date, which is EXTREMELY unusual for a Japanese game in that era, suggesting the "withheld from release" theory.

There is absolutely no doubt Sega bought the rights to Columns to capitalize on Tetris' popularity.