It does not mean Fewell was the one that agreed to take the job it was reported that April was pissed and felt overlooked that it went to Fewell, which was a big reason why he decided to opt out.
http://sports.espn.go.com/nfl/news/story?id=4797796
Halfway down the article Tim writes "Also factoring into April's decision was a belief he no longer had a future with the Bills after Dick Jauron was fired in November. April also held the title of assistant head coach, and felt snubbed when the team promoted defensive coordinator Perry Fewell to finish the season as interim head coach."
The Bills weren't interested in him and after getting overlooked he wasn't interested in the Bills