The entrant must specify a base style, but the declared style does not have to be a Classic Style. The entrant must specify the type(s) of fruit used. Soured fruit beers that aren't lambics should be entered in the American Wild Ale category.
This leads to questions like, "I have a fruited Berliner Weisse, does that mean it goes into 28C (Wild Specialty Beer)?"
The short answer is No. A fruited Berliner Weisse is a 29A Fruit Beer.
From the Introduction to Specialty-Type Beer preamble:
Specialty-Type Beer is a broad term used to refer to the styles described in Categories 28 through 34. They are different from what we call Classic Styles that are represented by Categories 1 through 27. The Classic Styles stand alone and can be fully described in a standard BJCP style description. Specialty-Type Beers involve some form of transformation of either a Classic Style or another base beer, either through adding additional ingredients, or handling the beer differently using an alternative process.
From the Category 29 Fruit Beer preamble:
The Fruit Beer category is for beer made with any fruit or combination of fruit under the definition...
So you apply those two statements, then any specific instructions in the style. Berliner Weisse is a Classic Style (style 23A), so it is a Classic Style that is transformed with the addition of fruit.
The statement about lambics is the exception to the rule, not the rule itself. The reason for this statement is that there is a 23F Fruit Lambic style, and that lambics with fruit should be entered there, not in 29A Fruit Beer.
The statement about soured fruit beers is for non-Classic Style beers. So if a beer without a base style is soured and contains fruit, that is what the 28C Wild Specialty Beer is designed to describe (among other things).
The statement about the declared base style not having to be a Classic Style is for beers with an actual style. For example, a raspberry porter can be entered there. The beer doesn't have to be an American Porter or an English Porter, just have general porter-like qualities. For a Wild Specialty Beer, the base style is less important and might not even exist. That's why a general description of the beer is all that is requested.
The point that seems to be confusing to people is that some Classic Styles happen to be sour. This is an irrelevant point for entering in the Fruit Beer style. Yes, Berliner Weisse is sour. So is Gose, and several other styles. Fruited versions of these beers go into the Fruit Beer style since the base beer is a Classic Style.
We understand how this can be confusing to people, so we will look to clarify the instructions in future releases of the guidelines. However, try to keep in mind that there are no tricks in the guidelines. Use reasonable judgment and try not to cherry-pick individual sentences and give them undue weight. In this case, a beer with fruit is indeed a Fruit Beer.