But it is not. Majority of the core game mechanics came from Alternity. The d20 System (both Modern and Medieval Fantasy elements) have derivitive work from both Alternity and DnD, but mostly from Alternity. The Flavor text might be DnD, but the mechanics is Alternity.
Also, without Chainmail DnD would not have existed. It was the seed idea that made DnD, therefore the roots start there.