I would say that there could be a problem finding relevant purchase invoices ie invoices for non-current assets. However, I don’t think teh test is entirely useless. The auditors oculds leaf through the file of purchase invoices and ensure that any for non-current assets found their way to the non-current asset register and to the budget.
That wouldn’t detect invoices that were missing. That’s why the answer suggests using goods received notes (though they could presumably go missing also).