Aksai had guessed it right.
He could not harm the Myriad Worlds Painting with his current cultivation base.
However, he never considered the possibility of the artifact harming him.
Although he had thought about the Myriad Worlds Painting being a 4th-order artifact or beyond, he abandoned that idea after pondering over it multiple times. After all, he had never seen or felt the natal spirit of the artifact before. Without the natal spirit, Aksai concluded that the secret dimension in his possession was a 3rd-order treasure.
Under normal circumstances, Aksai's assumptions would have been right. However, due to his lack of background and the fact that he had not really explored much of Dadangar, he couldn't imagine a scenario where a natal spirit was put into slumber by the previous owner.