To clarify, this is NOT emulation. This is a full on remake of Sonic 3 & Knuckles, which pulls the level data from the original carts in order to run the game in 16:9 mode. Think of it similar to Taxman's Retro Engine, but specifically for Sonic 3 & Knuckles.
In order to run the game, you must have a copy of Sonic 3 & Knuckles purchased from Steam (or elsewhere) to provide the art, level layouts, etc from in order to run this engine.