fastboot oem off-mode-charge 0 is the genuine method to automatically
power on Android when the charger is connected.
To be clear, it skips the screen where a charging battery is displayed.
Not all OEMs/vendors implement the command in bootloader.
Or on some devices it's reset on next reboot.
If off-mode-charge is disabled, bootloader won't pass androidboot.mode=charger
commandline parameter to kernel when charger is inserted, so device boots normally.
I have tested this command on a few devices and so far only the Nexus 4 device allows it. I need to buy a couple of devices where this command is working.
I you have tried on your device and it works, please give me your exact model.