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?

>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.

In this example, do you think the speaker of the quote was using it in a negative connotation? I think it's pretty clear that that is not the case.

Precisely. The speaker might happen to be "a bro" in the derogatory sense of the word (especially given involvement in an activity that may have a significant population of those people), but that's fairly distinct from simply calling someone "bro" as used in this quote. But I do say "fairly" because bros (in the derogatory sense) probably also call each other "bro" (in the sense that someone is close like a brother) at a rate that exceeds baseline.

I have definitely been greeted with "what's up, brother?" several times. And I took it exactly the same as when other people have greeted me with "what's up, bro?" -- the former came from someone a bit older than me, and the latter came from someone my age. Intent was 100% identical as far as I could tell.

That may have been the case, but it does not mean that all such uses are 100% identical.