I'm pretty sure he hasn't denied exiting Twitter via SecondMarket.
I'm pretty sure he has: http://news.ycombinator.com/item?id=2650977
That only denies SecondMarket - I think what he is saying is they might have sold to later stage VC/PE firms in an organized round and doesn't appear to be denying that he somehow exited. Honestly, don't blame him for at least cashing some out on 20X+ home run.
[1] http://www.usv.com/investments/