I hear what you're saying, axis (and CBFD); having
The Flying Krock be considered a 'world' does seem a bit of a stretch, doesn't it... I mean, there's just one level... no Klubba... only half of the helper Kongs appear, and if you think about it,
K.Rool Duel (that's how it's spelled in-game - no space after the dot) is essentially the boss battle we miss out on in the anticlimactic Stronghold Showdown... so I must agree that it doesn't match all of what you'd expect from a typical world in the DKC series.
It does have a unique name for the boss battle (unlike in DKC), and new 'overworld screen' with a variety (however small) of locations to visit... so it's definitely more of a 'world' than DKC's Gang-Plank* Galleon
(*in DKC, 'Gangplank' is hyphenated) was... but due to the lack of an actual boss in K. Rool's Keep
(yep, space after the dot this time, unless you're returning from the Lost World, strangely enough...), I'm certainly willing to consider the 'extension' concept.
The main reason I've got things laid out on the site the way I have is that it's the most simple and complete way I could think to organise the pages from a taxonomical/structural perspective. It does seem odd to designate a whole world number (7) to what is essentially a single level and a delayed boss battle, but K. Rool's Keep already has six levels (until DKCR came along, five levels and a boss per world was typical, six levels was a 'big' world), so adding a new
extension screen with additional challenges (plus save/transport points) is pretty extreme... but then DKC2 is quite the epic game, so that's kind of appropriate...
Hmm, it's a melon scratcher.
I have to say, I'm leaning towards the notion of considering The Flying Krock an extension of K. Rool's Keep - mainly because without this extra challenge, the Keep would be completely devoid of an actual area guardian/boss, which is unheard of in the rest of the game (and the whole DKC series, if I recall correctly). Besides, having a 'final' world with seven levels, a fake boss/surprise twist AND a real, tough 'final' boss feels more badass than having an extra world with just one level and a boss...
What does everyone else think? Shall we take a poll?