There's very little fluorine-containing anything in the size of cell used in wireless headphones. Note that the article you linked used many amp-hours worth of

There's very little fluorine-containing anything in the size of cell used in wireless headphones. Note that the article you linked used many amp-hours worth of batteries for each test, and they took multiple minutes to get to over 50ppm of HF. Scaling that down to headphone cell size (~600mAh) it would take tens of minutes to produce any significant amount. My theory is outer material containing vinyl decomposing to chlorine which caused the direct reaction. Would also make sense as thermal decomposition of PVC is only dependent on temperature and would be in direct contact with the person holding it.