You might be right. On the other hand, today they used a missile. Such a missile costs about $200k, while a Vulcan round is under $20 (collectors can buy one round for $20, see [1], the military can probably buy in bulk for much less).

Maybe they just wanted to show off. But maybe it's just not that easy to take down a high flying balloon with unguided munitions.

[1] https://www.gunbroker.com/item/968543844