Earlier quoted context omitted.
FYI, just because Apple said it doesn't necessarily mean it's true.
The CEO made a public statement. Any number of his employees or security experts could call him out if he were lying. You think all computer security experts are in cahoots on this? Cook could have lied and said building a special OS was impossible. But he didn't because he knows someone will find out. When you're facing a subpoena, you don't knowingly lie.
Not that I believe Apple's CEO is lying, however…
You don't knowingly lie if you think the risk of getting caught is unacceptably high. That last part is important, I think, and indeed, you state, "because he knows someone will find out", so I know you get it. However, I don't think we should forget that a great deal of the concern we have presently in this thread about the government's capability in introspecting our technology is indirectly the result of the DNI lying under oath¹.
¹I know your post says subpoena, but it seems similar enough that its worth reminding…