Actually I could verify (on one machine) that this value is indeed the camera time (interpreted only relative to next and previous values, like a host’s python monotonic clock) by recording a timer phone app in the camera stream. The jumps in the value of CAP_PROP_POS_MSEC across grabs perfectly coincides with the camera stream’s recorded timer app time that it is capturing.
By making a video of a phone’s timer app, these things can be verified (or, refuted on other machines and versions). If you have a video viewer that can go frame by frame, that is.