Lemme break that down a little. A man walked into a supposedly secure convention hall filled with non working weapons and filled with many figures who work in one of the protested industries as of late with a knife, cut loose a non functioning gun and was able to walk out past security with said gun.
IANAGunperson but from what I have read, in the US the difference between non-functional and kill-'em-all is often just the totally legal, unrestricted purchase of a replacement part.
I’ve seen lots of bazooka and recoiless rifles made safe by cutting a big hole in the side of the tube where your face normally is when firing.
Cannons are often made safe by filling or plugging the barrel with cement.
I honestly don’t know how they’d make things like non-nfa machine guns non functional, but I expect it’s often something where they remove the bolt altogether, or remove the hammers or striker etc.
Some parts are easy to replace. Others are very difficult. But the ATF usually requires something irreversibly damaging be done. It depends on exactly what the weapon is and how it works.