I don't think Lucas is actually meaning to deny that he -ever- thought about doing more than 6 films, I think it's just the story he was originally planning to take 9 films, ended up taking 6. So anything he came up for beyond episode 6 while the plan was 9 films got rolled into the 6th chapter, hence now that the sith is destroyed, Lucas hasn't come up with what would happen next.

In that 1999 Vanity fair article, he actually says the plan is no longer 9 but 6, which means at least at that point he was being up front about it.

I think when he says he never came up with anything for a third trilogy, he just means he never came up with a storyline to follow ROTJ.