Look, you've got the internet, you can look it up if you please.
Google : DX10 Supreme Commander Forged Alliance
Chris Taylor said it himself in a press release, and its on Wikipedia still I belive with a reference.
As far as DX10 being apart of the game design that was never introduced, thats possible - I watched the game develop over the 4 years, and waited a while before I bought it when I had a new system. So it is entirely possible, but from the info I had before purchase it diddnt seem to support only DX9.
Forged Alliance, for what ever reason, looks awful on my super-rig. It seems to only effect users on the max settings. A few players on the forums have been talking about it with comparison screen shots, and complaining about the slower sim speed.
Heck, I have a few fraps of the vanilla version and Forged Alliance version, and the difference is at least 6 years of graphical development in the industry.
I do hear you though, a lot of players say the graphics are better, but for me and others on max settings, it looks awful - just awful, and runs like a snail, where as the original doesnt do either.