There are optional worlds/areas in Kingdom Hearts you don't necessarily need to play through in order to beat the game such as 100 Acre Wood, Monstro, Atlantica, and Halloween Town - whether you did or not, Sora remembers them nonetheless. So if we apply the inverse, just because we aren't shown Sora's memories of Tarzan's world, that doesn't mean they aren't there, tucked away. You might even like to view it like this: perhaps Namine never tampered with those memories in the first place. Unlikely, but there's room for speculation.
Also, am I the only one who enjoyed Deep Jungle? Tarzan is one of my favorite Disney films, and I found surfing on trees quite awesome. It's like a KH exclusive world, just one of the reasons I like replaying the original so much.
i dont remember Monstro or Hollween Town being optional