Usually when an unreleased game is found, one of a couple of things occur. Most often it is found by a preservation team that has acquired it and plans on releasing it. Sometimes it is just a video of a title and that is all anyone knows. Then, more rarely, the original developers find an unfinished title on old hardware and release it. What we have with Overstorm for the Game Boy Advance is a mix of two of those options.
Yes, the original developers have found old code of an unfinished title for the Nintendo Game Boy Advance, namely a title called Overstorm. What we have here is an extremely detailed side scrolling platformer- it looks like it was created in the style of the Donkey Kong Country, Xperts and other titles, where CGI workstations created super detailed content and it is downsampled to the less powerful hardware. This is purely based on the graphical look, it just looks like the other games that used this method (which begs the question, why aren’t these super detailed versions ever released on newer, powerful, hardware?).
Now, how do you get your copy of Overstorm? By purchasing a game called Volt. Overstorm is a free bonus. Currently there is no way to legally obtain Overstorm other than to purchase Volt. Maybe in the future the developers will offer the ROM file for free without having to buy something else.