then you need to do more research on him. he's been in a a few bands, one of which received some acclaim.
Carrie Underwood has been a failure singing her own songs? Kelly Clarkson? Daughtry? all have sold millions of albums because they're good, not because of American Idol.
most of the people on the show already have a background in music. it's not like they show up because one day they can randomly sing.
if anything, this season in particular seemed to pick contestants who already had failed in their first shot to break it big and were back to scraping by all the while keeping the dream alive. AI provides that platform.
lots of people have made other musician's songs famous. if somebody is playing your music, and they can do it well like Cook, then that is the ultimate compliment and reaffirms that you're music is still relevant, no matter how long ago you created it.