From the article:
Lucas said the suspect then walked out of the store, where an off-duty firefighter held the suspect at gunpoint until police arrived moments later.
The suspect, a 20-year-old white man
So he was detained after leaving the store. The "good guy with a gun" may or may not have stopped a massacre but if the rifleman had intended mayhem, at least initially there was no one who would have stopped him.
And, what purpose does including his race in the story serve except to exacerbate an already extreme racial divide?
Last, IMO, I agree that, given the information available in the story, this person was attempting to demonstrate his right to carry. Not the best time nor the best way. And I will wager he will be charged with something. And I also will wager that the ACLU will not represent him.