"A perfectly balanced standing wave transfers energy in both directions at the same time, so there is no net energy transfer"....I dont quite understand what this means.
Its a matter of degree.
If either the source end or the load end impedance is mismatched to the transmission line impedance, you can wind up with a situation where all the power available from the source is not transferred to the load at the far end. The fact that there are standing waves present is a symptom of the mismatch; not the cause.
Even with a perfect match at the driven end, and only a slight mismatch at the load end, there will be "standing waves". The reflection coefficient does not need to be unity...I stand by that.
I'd just add that standing waves present is a symptom of a perfect mismatch, where the reflection cofficient's absolute value equals 1.