The "cement" used in tire tube patch kits (de)vulcanizes the rubber in the patch and of the tube. Which is a chemical process, usually using sulfur, where the rubbers bond and form a stronger bond than just an adhesive would do.
Rubber cement is just a gooey adhesive. Usually latex with acetone and other chemicals to make it more pliant. You wouldn't want to use it to patch a tube since it is not very strong, will degrade the integrity of the surrounding rubber and patch, and in general make a mess.