The regulation naturally can't be absolute, but it is enforced to the extent that the cabin attendants can enforce it. I got shouted at angrily for looking at an email on my cell phone while waiting to disembark from an ANA flight once...
This is true on Japanese carriers even outside Japan. If you fly ANA or JAL to the US, for instance, they will not let you use a cell phone on the ground while taxiing to the gate, even though US carriers will happily let you do so.