Libs I use has these methods . . .
The backlight is completely separate from and has nothing at all to do with the LCD controller.
This is why the Hitachi LCD controller datasheet and all of the other datasheets that are derived from that datasheet have no instructions to deal with the backlight.
The LiquidCrystal library is written to deal with the LCD controller and since the controller doesn't deal with the backlight neither does the library.
The instructions that you mention most likely refer to an library that is geared to a specific I2C adapter either by means of its inherent code or perhaps by means of a constructor. In any case, although it works with your hardware, it is not likely to be applicable in this case.