Personally, I'm glad you're asking questions and being upfront about your uncertainty.
I'll let others give the details on Xinjiang but I'll say, briefly, that China's work in eradicating terrorism and improving the standard of living for Uighur Muslims and others in the region are commendable. Not only should this be defended, it should be praised and used as a model elsewhere.
The answer as to whether I/we agree on that definition depends on a class analysis and also relates to your question about authoritarianism, which I'll answer first.
The class war broke out a couple of thousand years ago. It has been authoritarian since the beginning. Under the current system, dissent will be met by armed police and military force, made lawful by legislation and supported by the judiciary; the bourgeoisie will deploy this force at the first sign of any threat to its power. Peaceful protestors against fossil fuels, wars, and the regression of women's health rights will be locked in prison, for example.
Given these observations, the only options are: (i) ending authoritarianism (which is one of the ultimate goals of communism); and (ii) continuing authoritarianism in (a) the same form (which means continuing with capitalism) or (b) a different form (which Marxists think of as necessary in the transition away from capitalism).
If we then change the question to, 'Do you accept that socialists must be authoritarian?' Yes: https://www.marxists.org/archive/marx/works/1872/10/authority.htm. If the workers gain control of the means of production, the only way they will maintain control is by exercising a similar kind of legislative, executive, and judicial authority as is currently wielded by bourgeois states. I also can't see how it's possible to gain control of those means in the first place, except by being authoritarian.
Back to the definition of 'tankie'. First thing to note is that I've never called myself a tankie except in jest. I have no idea what critics mean by it unless it's a pejorative term for a Marxist. In which case, the definition, 'Defending the use of force against civilians expressing disagreement over their government’s decisions peacefully' is just absurd.
If tankie is simply shorthand for 'Marxist', then it refers to 'historical materialists'. Historical materialism is a way of looking at the world that treats everything as a process, a relation, as historically contingent. We can go into this if you wish.
That's not to say I don't have anything in response to:
Defending the use of force against civilians expressing disagreement over their government’s decisions peacefully?
But my response is to ask a few questions:
- Are there examples of liberal democracies using 'force against civilians expressing disagreement over their government’s decisions peacefully'?
- If so, does this make liberals (who by definition support liberal democracies) 'tankies'?
- What is meant by 'peacefully'?
- What is meant by 'government'?
- What is meant by 'use of force'?
- What is meant by 'civilians'?
- What is meant by 'disagreement'?
- What is meant by 'defending'?
- What is meant by 'expressing'?
I doubt very much that (m)any Marxists would support bourgeois states' use of force against peaceful anti-war and climate change activists. But the same people might support a socialist state suppressing pro-war, pro-fossil fuel, or pro-capitalist activists who are known to be supported by a foreign imperialist state's secret service.
Much of it comes down to class position. Marxists understand history as class struggle. It doesn't make much sense for a Marxist to treat individuals separate from a class analysis. So the individual 'peaceful' 'civilian' 'expressing' 'disagreement' is actually entwined in a much larger class struggle and is part of one class or another, which changes the dynamic of the question.
Feel free to come back with further questions.