Pedantically true, yes. If I hold a gun to the witnesses head and demand they testify or I will pull the trigger, no one is "forcing" them to do anything. They have the choice to refuse to testify and suffer the consequences.
It's the same with Manning. She has the "choice" to be thrown in prison because she refuses. Usually doing something under threat of deprivation of live or liberty is legally considered being forced to do something.
I can rob a bank at gunpoint and then argue that the bank "chose" to give me the money, since they always had the option to refuse, just as you are arguing here that Manning has the "choice" to testify. When there is a threat of deprivation of life or liberty, that is not considered a free choice. It's coerced, under duress, also known as forced.