That is the Baseball Reference ranking. He is tied with Mays (1964) and Ted Williams (1946)
Click here to see their rank for position players.
The only players to ever have a better season were
Per game (he played in 139 and in one game he was just a pinch runner), he had .0770 WAR. That would be 9th best ever. See Mike Trout's WAR Per Game Is Historically One Of The Best (So Far).
He also had, not surpringly, the best rookie season ever. See Does Mike Trout Already Have One Of The Top Ten Rookie Seasons Ever?
To see the top seasons ever in WAR from Fangraphs, go to Best WAR Seasons For Position Players-Fangraphs. (thanks to David Appelman) They give Trout 10.0 and that is tied for 83rd best every. But, recall he only played 139 games. The average games played for the top 30 is 150. If Trout had played that many and had racked up the same rate of WAR per game, he would have had 10.8, good enough for 43rd.