Firing rounds into the air is
NEVER a good idea, but doing what he did worked.
You can "suppose" till the cows come home but it doesn't mean it happened. Fact is he didn't fire into the sky, hit a rock and ricochet into one of his grandkids or have black gold come bubblin' out of the ground. Fact is none of that happened.
It is also a fact that the best training classes on the planet do not and cannot prepare for every isolated circumstance, and Grandpa may not have had any training at all. He did not "crank off rounds in a random direction to "scare" people", he fired one single round into the ground in front of himself (presumably) as a warning that he was armed with a gun and was willing to use it in order to get the bad guy to stop and comply with his orders.
JonM, what would you do if you were faced with a situation that was not covered in your extensive training (or perhaps a perp who has not been through the same training)? Call your instructor

? Wet your pants

? Or improvise, adapt and overcome

? Personally I would hope that I would do the latter.
Again, Grandpa apprehended the bad guy and held him without injury until the police arrived. I see nothing wrong with that. The
only thing that went wrong in my opinion was Grandpa's arrest.