when this sort of data can distinguish between walking normally and walking drunk, identify the sex of the wearer, whether they're carrying heavy objects, etc, yes, it can distinguish between playing basketball, running, and sex.

this data can be 9D (accelerometer/gyroscope/compass), it's much finer-grained than "moving a bunch or not", and it wouldn't even take those kinds of movement analyses to differentiate between "horizontal, active, and highly correlated with person B (estimate: sex)" and "vertical, active, highly correlated with person B (estimate: dancing)"