Those messages are not written and selected by JAL. It is messages that the authorities control. Why they have not added any additional messages is difficult to say, but already at check in the ask if you have been to China within the last 14 days, so there is less need to remind people again.
In Haneda there was special lines at the health check for people arriving from China, Hong Kong and Macau. So maybe at the early morning, they had yet to have inbound passengers from risk areas.