Yeah I knew abou this for a while now. It is considered the third pillar of their marketing (Wii, DS, DSi). I can only guess why though.
I think they are just saying that for marketing. So as to not annoy all those people who recently bought a DS lite.Besides if they just replaced the old one and people thought they were the same-a person could walk inro a shop and pick up a camera based DSi game thinking it would work on their old DS system.
It also does not put companies like GuitarHero totally offside because they are still supporting the system.
Maybe instead the DS lite would now be like a cheap model and then there is the more expensive newer DSi with a camera.
Because seriously the only main difference (in terms of games) is the camera and the Slot 2. Everything else is an aesthetic change that won't/shouldn't change game play.