Putting aside the quality of forbes, at the end of the day, the FB app is proprietary software - we don't know if it is using the data for tracking, and even if

Putting aside the quality of forbes, at the end of the day, the FB app is proprietary software - we don't know if it is using the data for tracking, and even if security researchers were to determine whether one particular version of the app were not using accelerometer data, there's nothing stopping FB covertly pushing a later update that enables it later (it wouldn't even need to change the permissions). FB is a company that doesn't deserve the benefit of the doubt, ringing the alarm bells is perfectly justified.