I get that it is an abbreviation, but does it have new meaning? What meaning or connotation would be lost if a user of the word said brother instead of bro? Would there be confusion?
No, but it would be unusual. You don't hear people using a term like "crypto brothers", for instance. "Bro" carries a different connotation, and it isn't a positive one in this example.
I don't think it is simply an abbreviation in this case.