I seriously doubt anyone would misremember such a dramatic event. Details, maybe but not something so important as a date like this.
Most likely, is that the scrawl was done post eruption....should have been plenty of charcoal laying around.
Other than the date, I wonder what else is in that scrawl. Probably trying to read the message with different spectra will yield some results — unless the guy writing it was only able to scrawl the date before the eruption killed him. :^)
http://www.freerepublic.com/focus/chat/3697309/posts?page=9#9