That's the claim. And if Apple were to make that tool, how likely is it that it would be used once and only once? ("well, ok. i promised.") How many other agencies, or governments won't demand that Apple do it for them, too?
Knowing that such a tool could be made, how long before some other state agent recreates it? For that matter, how much money would every single person associated with developing the tool be willing to forego? ("Riches beyond your wildest dreams!" "I don't know, I can dream some really big numbers.")