My understanding of what is going on is that the motion says there's no debate, which means that no member can get the floor except by a point of order.
You can't move a subamendment off a point of order, which means you can't move the subamendment because there's no debate allowed and no having the floor except on a point of order. The job of the chair now is to put every question forthwith and successively without debate.
You can't get the floor, so you can't move a subamendment.