I'm a little confused as to what distinction is being made here between "provable" and "unprovable" factoring algorithms. Is it just that "provable" ones may output the correct result all the time, while "unprovable" ones are probabilistic and output the correct result with high probability? (E.g., roughly the distinction between P or ZPP on the one hand and BPP on the other hand, if these hypothetically ran in polynomial time and we were talking about decision problems.) Or is something even weaker meant by "unprovable", like retuning possibly incorrect results based not on a random input but on the number to be factored? I assume it's the former -- if it were the latter I'd say such things are not really factoring algorithms, practically useful as they might be -- but the way it's worded isn't exactly clear.