The reason why you see Crysis 2 Sandbox/Crysis 2 Devmode videos on Youtube is because they are using them from the "leaked" developer version of Crysis 2. It contained both of them, however they, as well as the game, are buggy. The Sandbox crashes often (sometimes when making a new map, sometimes when loading a new map while you already have one loaded, and other stuff) while the game has graphical issues (graphics tearing, sometimes textures are weird) and it crashes sometimes as well.
Technically it's illegal to have it since it's a build meant for the Crytek developers only (As evident by the "Crytek - Internal Use Only" freeze that occurs after you finish the Sub level. The freeze occurs right before the Intro that shows the Nanosuit) but that doesn't really stop anyone from downloading it
Crytek said they would release Sandbox at a later time, I believe they said Summer but don't hold that against me. The console (as well as modding attempts) was locked down with the release of Patch 1.2.0. Because of this, the game no longer looks at the "gamecrysis2" folder and instead looks into the .pak files instead. As I said before, the console, as well as certain variables (like con_restricted) were locked down, and those values cannot be changed.
Perhaps at a later time it would be unlocked, but only time will tell...