Once again I will state that theory crafting about development to prove a point either way is stupid. How do you know that by not "cutting anyone" he isn't talking about all three pokemon being separate characters? If all three pokemon were still in the game, I wouldn't count that even close to a cut.
Brawl was released in 2008, a time where the most recent titles where
DK Jungle Climber and Donkey Kong Barrel Blast(2007). Both of these games where not received that well and neither of them have anywhere near the hype/sales that Tropical Freeze had, meaning much less relevance for dixie then in comparison to today.
Dixie not being shown to coincide with Tropical Freeze as a negative factor against her is a FAR reach in my opinion. Why didn't they show Shulk at E3 when his game was at the hypest point so far? We can both make assumptions all we want but it is futile so I am done arguing about it.
Dixie is a threat. She is the only legitimate character for the 3rd DK slot besides K rool. Just strictly based on who could fill the slot she by default has a 50% chance barring no other factors. Seems threatening enough for me.