Thats not true. Playing out of formation is NOT an illegal formation.
Well I've certainly played against teams who have an otherwise legal formation, but have a player out of position and they've had 20% possession.
FC Bristbury
Season 5, Level 5
League Winners Levels: 1, 2, 3 & 4
Champions League Winners Levels 2 & 3 Double X 2 (League and Champions League), Levels: 2 & 3