That's not really how any of this works. It's not the defense that has to "support stronger facts", that's the prosecution's job. The existence of the metrocard evidence pushes the prosecution's burden correspondingly higher.
Now, here's a few ways this could've played out:
Scenario A is that a random guy with a couple of drug priors is actually trying to keep his life together, leaves work, goes from work to a place to cash out his pay check, goes hanging out with his girlfriend. He somehow ends up wrongfully accused, possibly because of his priors.
Scenario B is that the guy was walking around town, randomly ran into the victim (while presumably having been snitched on by said victim), and, after a verbal confrontation, murders the presumptive snitch.
Scenario C is that the brothers had just heard the guy snitched on them, they're fuming, and, as a spur of the moment thing, go on the prowl for the snitch, and kill him.
Given the existence of the Metrocard evidence, B and C are off the table. Instead, you have scenario D:
Random guy with a couple of drug priors but no prior history of violent crime plans and executes a homicide, devises a sophisticated alibi that has him at reasonable locations at reasonable times while biking to the crime scene and back. This perp was sophisticated enough that there is no evidence pointing at him other than eyewitness testimony, yet he was still sloppy enough to confront the victim in public.
Scenarios B and C were fairly run of the mill situations, and eyewitness testimony might've been enough to clinch it, but D is the "this shit only happens on TV" type of crazy, and requires a correspondingly high amount of supporting evidence. This leaves us with A being the overwhelmingly likely alternative.