We have already talked about this a lot I think.
You cannot logically exclude an unreal (“square triangle”) from the description of a real (“a shape”). The statement “this shape is not a square triangle” would be an absurd statement - because such a statement would mean “this shape can be anything other than a square triangle” which is a statement devoid of meaning. The listener would be none the wiser for having heard it.
I guess you have not convinced an Excel Spreadsheet not to display a referential integrity error for using an imaginary reference in a formula. Try it once.
So why do you assume the Buddha would have not understood that he coud only describe something as anattā as long as he was not assuming the attā to be unreal.