And even if it wasn't illegal, the number of midfielders gives you possession. The highest, the better. You can sacrifice possession if you ain't that good and you want to use long passes with a defensive lineup, but if you're better than your opponent, more midfielder allow you to keep control of the match.
As I said, illegal or stupid. Turned out it was both illegal AND stupid.

How could you do well in previous leagues? I'm really curious.