That's correct. DF did not consider bandwidth or ROP's which will be a factor in PS4's favor, but even the results they came up with are pretty huge. The difference between 1080p and 900p will be enough for most to choose PS4 versions.
While it is possible, I doubt we'll see any 900p version games, most likely they'll reduce framerate to keep it at 1080p or reduce resolution to 720p. Or perhaps worse case? scenario, they'll reduce draw distance and other effects. As far as I'm aware, they don't make any 900p televisions, only monitors.