I think that's just easily narrative effect. It seemed explicitly like a reskinned Blue Dragon Wyrmling. A sphere of annihilation isn't a ranged beam that needs to recharge that fires from an enemy: it's just an item. A sphere also annihilates whoever's holding it without some rolls, which I don't believe Adam was making.
You're right that it does 4d10 damage, but I think the CR 3 comment, that it fired as a beam (IE a line, just like a Blue Dragon's Breath Weapon) and the recharging on top of also being 4d10 make it more likely to be a repurposed dragon breath weapon.