Can wax really improve the performance of a FM transmitter?
See how crudely they strapped the conductive case of the battery to the bottom of the PCB , I would guess by "improving performance" he means "improve insulation to the bottom side solder joints" by preventing shorts yet adding stray capacitance to the battery case as a ground plane to reduce effects of crosstalk.

Don't take this concept as a good design practice nor the use of wax beyond what you already know.