Yep, they are batteries.. Lead acid batteries. They ABSOLUTELY can start a well tuned motor, there are 260hp jet skis running those batteries. My snowmobile has one of those tiny batteries, and it is a 4 cylinder/4 stroke 150hp engine, that starts at -20. A popular mod in performance auto circles is to replace a full size car battery with a sealed lead acid battery, aka the exact same one inside the jump pack.
But... See post #2. (mine!) It won't work because of the really crappy connection, not the battery size.