He wasn't describing the Golden Company, he was describing his own men. Sometime in either Season 6 or 7 it was mentioned that Euron cut out all of the tongues of the men on his ships. It seemed to be a myth but apparently it's true of the men on his main ship anyway (lest he was just joking).
There is an argument that she could have simply kept them prisoner, if for no other reason then as strategic assets when two of her allies had been captured. But at the absolute minimum, she definitely should not have burned them alive with her dragon as the form of execution. As many have said here and like Jon, Tyrion, Jorah, and Barristan told Dany, the optics alone of burning non-combatants with dragons is terrible for the supposedly non-mad daughter of the Mad King.