The effect of using a stool to raise up the legs to simulate a squat is not the same as the effect of the natural squatting position.
The reasons are that in the natural squatting position, the full weight of the body -- coupled with the pressing of the right thigh and left thigh against the abdomen (and cecum on the right side of the lower body and lower colon on the left side of the lower body respectively) would result in easy, quick and complete elimination.
Using a stool may seem to be equal to the task, but in truth, it is not the same as the natural squatting position.