Flutter has his drangonflies who helps him and there is a lot of means to hit someone even if he is flying, especially when the pilots can't even follow you with their eyes and can be killed with only one move imbued with Nen, even by just throwing a rock. Ants bodies are not necessarily harder than iron( they are just more durable than regular) humans and Bloster's attacks haven't been described as particularly strong but there is no doubt he can destroy a tank with bare hands, Bungee Gum isn't able to destroy a tank but Hisoka can easily do this with Shu or a punch.
Pitou used Ming Jol ik to control the army, she wasn't controlling all humans, she used only one of them to control the mass, it's the reason why they announced on the News that they should not go out, it would have been pointless if they were controlled. And even if they were, they wouldn't necessarily be weakened because they are not Nen users with high battle capacity like Kaito, they should even be stronger since they would be more focused on one goal. Kaito was weak because his Nen was sealed by Knuckle and Shoot and also because he couldn't fight like the strong fighter that he was, his movements were mechanical in order to train low level soldiers, who are still way stronger than normal humans( and even with that level Knuckle said it would be difficult for Gon, who has reflexes superior to normal to dodge them) a normal human wouldn't be weakened by that.
I thought the fist "faster than sound" was a bit dumb at first but it was just a pure physical feat. I guess it shows he was one of the only ones being able to do this in his era and maybe he didn't even know Nen, the fights didn't seem to be Nen battles and he acquired that speed by pure physical training. A lot of people can dodge bullets easily and Killua can dodge a knife at point blank even in his sleep. Leol's attack( a squadron leader) is said to be supersonic and Morel dodged it.
You say US army can beat them with coordination but how are they supposed to move with coordination when they are not even able to react to their attack, their weapons can't resist to Nen attacks because you can only counter Nen, with Nen( Conjurers use conditions for a reason). An army is supposed to fight another one, you really think their coordination is so good they can focus on a single moving target together? I don't think it's possible. They don't even have to run and hide, a guy like Killua can just rush and blindly destroy them, he has the reflexes and the speed to dodge absolutely anything. It would be like how Meruem defeated Knuckle and Meleoron, he reaches lightspeed and they have no time to react. When Killua passed the exam, they couldn't even understand what was happening and now he is even faster.
You can say that HXH army is not as competent as US's one but the fact is that HXH technology is similar to our world. So the army is not really that different, Killua beat an army so it's reasonable to think he could do that to US army.