Or more accurately, "ID" is testable, but "supernatural ID" isn't, and neither is "unspecified ID".
We *can* test for for the intervention of intelligence in such things as coded messages, whether a particular pathogen is naturally occurring or engineered as a weapon, etc.
But the IDers want to have their cake and eat it too. They want to pretend that "ID" is a science without allowing anyone to pin down anything about the identity, motives, or methods of the hypothetical creator. This is what I refer to as "unspecified ID" above. And you can't test for the products of design if you refuse to make any conclusions whatsoever about the design goals or methods. The *reason* they don't want to be pinned down is twofold: 1) They think it's God, but don't want to be outed as creationists, and 2) whenever ID *has* made a testable (i.e. falsifiable) proposition, it has been quickly falsified.
The creationists, on the other hand, admit that God is their idea of a creator, but this puts them into the "Supernatural ID" category, and that's just flat-out impossible to test for, because of when "anything's possible", nothing can be tested for or against. You found X? Maybe the creator *wanted* it there. Or maybe he had nothing to do with it and it's a natural occurrence. You didn't find Y? Maybe the creator didn't wnat it there. Or maybe it's not there just by chance. Or maybe it's there and the creator is hiding it for his "mysterious ways" reason. NO observation, of any sort, could decide anything for or against the proposition of a supernatural designer.