Because of the last part of that, it's meaningless. "Varies according to ... variety" means that it's simple to state that DWKs don't have to meet that definition.
Actually, no. Their size varies, which means it is different, according to age and variety. DWKs stay small all the time.
Er, why are DWKs staying small problematic? That sentence means that true dragons, as a category, have different sizes, depending on their age and variety. Jumping from that to the idea that
each variety must change as it ages is as big a leap as saying that an individual dragon changes size at
each age category.
You can have a table with Age as the columns and Variety as the rows, and finding the size requires navigating to the appropriate cell. DWKs just happen to have Small in each column.
I don't think that DWKs are True Dragons, I just don't think this contributes to the side I'm on. This thread has also convinced me that Incarnum dragons are not TDs either, but meh. That's largely because this thread has convinced me that the word "rigor" was foreign to the authors' minds when writing about true dragons.