It is possible to imagine that any collection of people could be mistaken or deceptive. Yet a barely conceivable possibility is different from a serious evidential alternative. Not every scenario expressible in words undermines justified certainty.
One might speculate that hundreds of independent witnesses happened to fabricate the same event without contact. Such a speculation needs relevant evidence before it becomes a serious competing explanation. By contrast, finding that all witnesses relied on one institutional statement is a concrete issue that should be investigated.
In the classical argument, certainty does not require that the mind become incapable of forming hypothetical questions. It requires that no relevant and grounded doubt remain against the established fact.