It's quite a stretch to say that the 6870 is "supported". It's more accurate to say it works under Lion at the moment, with some problems, and may have more serious problems at any OS update.
Indeed, you are correct. Which is why I ordered it from Amazon and when Lion comes out in the next couple of weeks should it kill the card, I can return it within the 30 day period. Yaaaaa! If not, I shouldn't have any trouble selling it for darn near what I paid for it and buying another card.
I hope that the drivers being in the Lion betas are a sign of that they will work and hopefully the drivers will work under Lion either way.
We shall see.