I think it's valid to think that he has no idea what is on the 'video' and won't say that it's not, because he doesn't know and won't say that it is believed to be, because if there is none or it's not, then he fuged up.
He could also just say "I don't know". He did state plainly that he had not seen it.
The one logical reason I could see (other than waiting to get paid) why someone would sit on this could be for legal repercussions. If this does indeed show Bryant doing something for which he could be arrested, you'd have to be extremely careful what you did with it. The police aren't big fans of people who have evidence of a crime and don't share it.