Well, it would be correct if they were using "average" to mean "median".
It's also correct if you know that the data fits a normal distribution, or something similar.
So yes, it's not, in general, statistically impossible for most people to be above the mean on some metric. But there are several interpretations of that statement in which it is correct.
Even if the person meant "median", the author's sentence is still ambiguous: It is definitely possible that 70% of _college students_ are "above median" at multitasking than the _general population_.
It's also correct if you know that the data fits a normal distribution, or something similar.
So yes, it's not, in general, statistically impossible for most people to be above the mean on some metric. But there are several interpretations of that statement in which it is correct.