I play Czerny every day. What he says about improvisation should be taken seriously.
Remember, this is the man who studied with THE greatest piano improviser, Beethoven, and the man who taught the OTHER greatest piano improviser, LISZT.
If Beethoven, Czerny, and Liszt don't offer you 'real' enough improvisation, maybe you had better listen to jazz instead!
Ajspiano, make sure you follow Czerny correctly-he intended his studies to be played at sight, WITHOUT looking down at the hands.
One of my goals for this year, among others, is to work on each and every single study by Czerny. Granted, not all of them are of equal value, but plenty of them are wonderful little works in their own right that would make for exciting concert pieces given sufficient preparation.