A review said musicians shouldn't be actors...

He did score third place on the first season of Deutschland sucht den Superstar, which is basically the Germanized version of American Idol. He also had a longer-running musical career than most of the people who won first places in the later seasons. So, yes, back in the early 2000s he was a musician. Not a very good one, but still. He was also quite popular back then, which is actually one of the reasons why this movie exists, it was rushed into the market to milk a cash cow that by the time the movie was released was dead.