I think bro is always an abbreviation for brother, but sometimes the act of abbreviating is done to save space (like company names) and sometimes to specificall

I think bro is always an abbreviation for brother, but sometimes the act of abbreviating is done to save space (like company names) and sometimes to specifically impart informality/metaphor (like TFA). But that doesn't mean the unabbreviated version can't be used metaphorically as well.