I feel like whoever first started bringing the term "assault weapon" to gun debates really killed the argument

That would be the pro gun control side. They wanted to conflate assault rifles as in the actual military rifles, and the downgraded civilian semi-auto rifles. The distinction is important, look up the process it takes to purchase a machine gun in the US sometimes. They deliberately want a culture of ignorance around guns, because the goal is total disarmament, not effective regulation.

You can see the result in this thread and others. People will claim that someone can just walk into a Walmart and buy a machine gun. Politicians talk about banning "fully semi auto assault weapons". The OP image and plenty of comments here mock the idea that someone should expect a base amount of knowledge in the subject before proposing new laws. Someone trying to define proposed regulation or correct a mistaken assumption about current laws is branded an "Ammosexual".