I agree that the 'more mathematical' one supplies "an instruction regarding which clue to ignore" (and that is what I have had written in from the very beginning), nothing in the preamble says that the (5,5) phrase is to be an instruction. It says that the (5,5) should "resolve the ambiguity". In the contents of the grid, the alternative phrase is apt given where on is to write the convergence.
If someone correctly filled in the grid, including the pair of numbers, and wrote the alternative phrase below, I think it would be unfair to mark them wrong.
As to why it needed to be a carte blanche, there would not have been much mystery as to which clue to consider ignoring if the bars were presented. You would know immediately whether it was an across or down clue, for example, and a few cold solves would give enough info about word lengths to narrow the search.