Autonomous means that it is able to act on its own.

E.g. suppose that at some point organization members decide that it should pay do monthly payments to some address to pay for certain work. A DAO noted this decision and started to execute it.

At a later point members might change their mind, they no longer want this payments to be made. But DAO will keep making them if there is no way for members to cancel this execution.

Thus DAO members do not have a complete control over the thing. It is autonomous.