From the way they are describing it I have to specifically white out each cell.
"In the box on the right, type =ISERROR(reference), where reference is a reference to the cell that contains the error value."
When they wrote "reference to the cell that contains..." it means, on a gramatical level, that I have to specifically refer to the cell I want to white out without any degree of speficicity.
I'm guessing that this is just sheer inability to express themselves as it seems a very complex way of just changing text colour to white - I have tried doing what I thought they were suggesting (for the whole column/workbook/etc selecting a conditional formatting of formula is =ISERROR(#NUM!)) but that just changes everything to white so clearly they are both not explainging themselves well, as I haven't understood, or they are just being strange.