Glad you liked it!
I assume Jack heard about it via Torchwood and promptly interrogated/seduced enough people that he'd know practically all the details - invasion, new appearance, hand severing and regrowing - but wouldn't have seen the Doctor himself, so wouldn't be able to recognise him.
I'd also guess that Jack knew about the hand severing before he found the hand, because strange as Jack is, I'm not sure even he would just randomly collect hands.