People who throw the word “tankie” around indiscriminately aren’t using it right

There's no right way to use it, because it's a completely meaningless term which only serves to discriminate and isolate those who support more radical views, such as a defense of revolution against capitalist dictatorship.

it applies specifically to extreme communist fanboys who are apologists for communist militarism

There are "military apologists" among the conservative population which admires the Soviet Union because of its army. Are they tankies as well?

I see this as an aversion against violence in general, common among liberal "leftists". Someone who is deserving of the label "communist" simply don't reject violence as a way to fight against the capitalist system, which is already a very violent system. Communists do not support unjustified violence, but they simply don't condemn it in a revolutionary process. All successful revolutions were a violent, brutal process, to defend an abstract non-violent revolution is simply falling into idealism and losing sight of reality and history.

Tankies will say that the Tiananmen Square massacre was either justified or didn’t happen.

As what liberal leftists would call a "Tankie", the Tiananmen Square protests did in fact happen. What didn't happen is the army gunning down on students and civilians. There is not a single footage of that happening. And even eyewitnesses of what happened there, like a Chilean diplomat whose cable has leaked in WikiLeaks for public view, they all say they didn't see any army member shooting people. They saw hurt people, bleeding people, and even army soldiers being killed and burned down. A lot of people were hurt in the midst of the confusion of protests. But not a single army soldier shot a single student on that day.

What you don't know about Tiananmen Square protests is that while some policies of the Communist Party of China generated a dissatisfaction among the younger population, the protests were largely financed and fueled by the UK and US, especially by the "National Endowment for Democracy", an organization specialized in recruiting and training leaderships of mass protests, used for regime change operations through color revolutions.