I'd like to point out a few things here, that I think are a little off in your assessment of the situation. (Edit: All of this is assuming that your Skype is used for official work)

First

It seems like you think that a joke on concentration camps (Especially one downplaying them like calling them an education camp) can be equated to a silly joke like "Where we are going, we won't need roads". This is not the case at all. One is harsh and should always be kept to a more private setting where you know it won't offend. The other is... well, just silly, non offensive. With humor it is always advice to use non-offensive humor in work-settings, unless you specifically know that harsh humor is accepted by everyone at your work place.

Second

You talk about the relaxed setting at your work with rubber ducks. This is great and shows that humor is acceptable. Keep in mind that rubber ducks are also innocent in nature, so it doesn't show that everything goes, just that humor like "Where we are going, we won't need roads"is fine.

Third

You talk about human rights and US citizens. This has nothing to do with the situation really. Because even if you are a US citizen, you can get offended by stuff. Someone obviously did here, the human rights act of 2019 has nothing to do with this.

Fourth

You say that your workplace Taiwan as Taiwan. This is fine, but keep in mind that there's no reason to assume that the person was offended by the pro Taiwanese opinion. They might as well just be offended by the joke because of it's dark nature.

Fifth

You mention your manager didn't even understand the joke as if that means she can't ask you to stop. Someone complained about a dark, out of place joke that was political. She probably knew there are no rules specifically stating "No dark jokes" but there's one saying "keep political opinions to a minimum" so she took the route that the rules allowed, to help this person.

TLDR:

You made a joke that was not appropriate. It doesn't matter how many arguments you try to find that supports it being okay, because your manager told you it was not. Keep your humor more light and innocent.