It's unclear what was posted, because the account claims it was to attract attention to the abuse, not to show it, so systems for detecting known such content would've detected it as a frame of a video of known such content, but it doesn't mean the image itself showed such activity.
I'm not saying this in defense of anyone, as the account in question is a troll who posts anything for attention, Elon's the same. But the media is also the same.
Musk says no one outside Twitter saw these tweets, but WaPo categorizes them as "child sex abuse" just by assumption, because it'll attract attention to their article.
I'm so sick of this society.