The Oprah interview will be shown tomorrow so I don't know yet what Armstrong said exactly.
The trouble is that if he admits doping, he risks facing several law suits with every dollar he's got at stake. (according to my newspaper, his estimated wealth = about € 100 million dollar).
So, how can he admit guilt while facing that? Some time ago, the USADA offered him a reduced sentence if he was willing to talk openly about doping. He didn't agree so they gave him a life-long ban.
Throughout his career, Armstrong has been protected by a number of guys: Pat McQuaid, Hein Verbruggen, Johan Bruyneel,...
The theory is that in return for more leniency towards himself, he'd be willing to nail his protectors against the wall.