Look it's pretty simple. there are currently 4 confirmed M cars.M3, M5, M6 and the Z4M.
The only rooms for confusion here are that he mistaken the 6 series to be a 5 series coupe or he has come across the rare and legendary Z8. The M6 shares the same engine as the current M5, while the Z8 uses the V8 from the previous M5.