Tested in IE 11 and FF 45.0.1 with same results.
Follow demo link here:
Click the Disable / Enable Quantity Column button.
It appears the cells in Quantity Columns are disabled and user can't type anything on them.
User can press CTRL+C CTRL+V to overwrite the cell value.
Also Dragging the Cell "Fill Handle" will overwrite the cell value
As a side effect, once a cell has been pasted into a disabled cell, the disabled cell becomes editable (even if you paste copying from an already
disabled cell which also means that the disabled attribute is not copied/pasted over)