I saw a guide that said once you finished the game, the rocks blocking the door in the cave in the jungle level would be gone, but I've finished the main levels and the bonus levels, and the rocks are still there. Is there something else I need to do?

That only applies to the switch version, since here on itch.io the easter egg in question is playable on its own in the form of Kiwi 64

Oh, okay. Thanks!