I was litterally just linking a source to a post I read a while back. It wasnt debunked when I read about it, in fact people had accepted it. But you are correct.
Retro Studios intended to give Metroid Prime 3: Corruption larger environments than Metroid Prime 2: Echoes, and enable the game to run at 60 frames per second. The developers were also interested in using the WiiConnect24 feature to provide additional content for the game that would be accessible from the Internet. A small Metroid Prime 3 Tech Demo was shown at E3 2005, created with the MP2 3D Engine. (...) In January 2012, thanks to a fundraising organized by some users of the Assembler Forum, a 2006 demo, which documented a beta version of the second planet of Metroid Prime 3, was dumped and released to the community. The proto was developed to run on Gamecube hardware with 128 mb of ram, so it’s playable only on a custom version of the Wii emulator Dolphin. Using the debug menu it is possible to active/deactivate the various power-up and to try the third-person camera. The map is still incomplete, so only a few areas are accessible. Apart from minor differences, the general layout of the location is remarkably similar to the final version.
~http://www.unseen64.net
Unless they magically made the Unreal 2 Engine look exactly the same as their modified Metroid Prime 2 engine within a year, it's not the Unreal engine.
They have never openly stated so.Didn't was confirmed like a month ago by Retro Studios the the Prime trilogy ran on a custom Unreal Engine?