Yet with several witnesses present at the gas station providing testimony, he's been officially charged with second degree murder and had bail denied.
We don't know that yet at all. This is pure conjecture on your part.
What can be DEDUCED from what we have is that he provided a statement saying self defense. The investigation has statements from the kids in the car stating that they didn't threaten him.
Based on those two conflicting evidence statements, there is ground to charge him from only that.
While that is enough evidence to charge him, it's certainly not enough to convict. Not in the least. As there are other forms of evidence which would be other eye witness testimony from 1st hand witnesses not affiliated with either party in the incident, video footage, and other forensic evidence at the scene or possibly from elsewhere from the scene.
Again, if Dunn claims SYG, he'd be free right now so long as there was not an overwhelming burden of proof that an SYG claim could be negated. Sworn testimony from the 3 others in the car stating they did not threaten him would not constitute overwhelming burden of evidence.