From the text, feels like user agent should be capable, it should let site request sensor data and randomly reject certain requests after X (random) seconds, while providing fake data in other cases.

Half of the point of these scripts is to detect headless browsers. Most of them are fairly obvious, and even when they’re hidden, it’s things like what the article mentions that gives them away. For example, headless browsers can’t respond to permission requests, so they’ll likely immediately accept or reject the request for motion data.

Just record couple hours of phone usage IMU data and then feed it to them with random segments added together and they won't by any wiser. Or just rock the phone in it's cradle. There are companies with robots tapping the screens, adding some rocking movements is not going to be that hard.