That's true. He did join the Lions -- as in the past tense -- last week.
The Lions picked up a pair of running backs yesterday after rookie RB Mikel Leshoure was lost for the season and Leggett was waived/injured to make room for them. That waived/injured designation means he wasn't healthy (by the Lions standards, at least) so I'm not sure what happened.
So by my count, that's just three Chiefs that have left the team that have signed contracts elsewhere: