According to another respondent, they had a photo "taken" from the exact moment they scrolled by an ad. The author of the tweet confirmed the same happened to her in the same thread: https://twitter.com/PickledBoba/status/1338658784391000064 . That's quite strange. Even if we presume Twitter is doing everything aboveboard, why would they activate the camera when the user is not taking a picture or streaming?
They are likely computing the target of the user's gaze. It's valuable information for twitter and advertisers. Did the ad draw the user's attention?