Very good question and yes that's correct and true be it SCT or CDK, or modified SCT such as the EdgeHD or the RCX/LXR... and is the apogee U16M not big enough for you
?
The reason you and others are confused is - there's a lot of confusion about what exactly "flat field" means.
In short, it means
"The surface of best focus across the image lies in a plane."
For the EdgeHD, "flat field" means.... "small, round stars across a large, flat ccd". The EdgeHD has been design with this in mind.
This is important way? Because CCD's are getting bigger....
Remember NO optical system is perfect. Not even my Planewave CDK. It just does a better job than most; likewise the EdgeHD when using a large flat CCD such as a Canon 5D or Apogee U16M.
For example, you can still have a flat field that still has "coma" and "astigmatism". - In this case the stars will look terrible as they went out from the centre of field but it would still be a flat field.
Like wise, you can also have what "appears" to be a flat field with round stars all the way across it - However, it is in fact a "curved field". And when you change focus the stars will get smaller in one area and bigger in another.
Checkout Celestron on Flicker or celestroniages.com or some of results with the HD11 and ED14 Apogee U16M CDD.
cheers
T.