A chopper power supply still receives power from the mains in 50/60 Hz bursts (100/120 half-sines per second).
But even with a storage battery, the charger will have a 50/60Hz input.
One thing you say is correct: You, with no experience, don't know why the power supply uses a 50/60Hz transformer. Those of us who are experienced in the field have offered some insight. Yet you say we are wrong. Please explain how your lack of knowledge gives you this advantage.