Actually, I think you meant it is too "thin" to use the conventional slats as seen on other airplanes. The 400's wing is very thin. It looks razor sharp when you look at it from the front (as opposed to slower planes like the 75 or 76).
Keep in mind that the 400's MMo is .90, and boeing took it well past that during certification. They were able to achieve that through the highly sweptback thin wing design.
I don't know if the classic wing is also as thin as the 400, but it seems to me to be thicker when looked at from head on.
