And after skimming the paper, the only thing I could find in response to your point is:

Coercion detection. Since our aim is to prevent users from effectively transmitting the ability to authenticate to others, there remains an attack where an adversary coerces a user to authenticate while they are under adversary control. It is possible to reduce the effectiveness of this technique if the system could detect if the user is under duress. Some behaviors such as timed responses to stimuli may detectably change when the user is under duress. Alternately, we might imagine other modes of detection of duress, including video monitoring, voice stress detection, and skin conductance monitoring [8, 16, 1]. The idea here would be to detect by out-of-band techniques the effects of coercion. Together with in-band detection of altered performance, we may be able to reliably detect coerced users.