The body that Jean grabbed the 3DMG gear from was just that - a body. The reason we don't have scavenging in the game is 'cause the general consensus is that most people are pretty badly beaten up when killed by Titans (chomped, crushed, smashed, that sort of thing), thus the gear will probably be in an unusable state. It only stands to reason that there was a good chance that Jean grabbed damaged 3D gear. If anything, he was really lucky.