For example. Selam G., the author of the article that sparked the controversy said:
""""
[...]
…and then he says that an enslaved child could, somehow, be “entirely willing”. Let’s also note that he called a group of child sex trafficking victims a ‘harem’, a terrible word choice.
[>] We can imagine many scenarios, but the most plausible scenario is that
she presented herself to him as entirely willing.
[...]
""""
The only opinion stated here by Stallman is that "it's the most plausible scenario", but it's an objective truth that it's a possible scenario (even if not the most plausible) for the underage girl to be coerced into presenting herself as willing, given the public knowledge of the situation. Then, Selam gets angry because he stated that possibility.
Note that she doesn't disagree with it being the most plausible scenario, she disagrees with it being a possibility at all (an objective truth, because coercion is a thing), and then she twists Stallman's words (presenting oneself as [something] vs being [something]).