This is true.

It is much easier to get a proper Android build by using Eclair over Froyo, and Froyo over Eclaire.

Why?
Sources!
The Nitdroid dev is actually borrowing code from the AOSP and several android phones (including stacks and drivers) and meshing the ones that work together.
If they started with a new kernel, used the 2.3 AOSP, it is likely (certain?) that most of the "fixes" will break and will require fixing or replaced with code that is compatible.

So in short: Gingerbread = slower Nitdroid development
(it's just the way it is, unless Nokia provides the linux-derived hardware drivers for each component)