It probably doesn't even count as that, more a (proposed) wiki convention. (though maybe when Stanley is emperor of the universe, he will decide that that battle was the turning point).
AW stands for "Anno Warchalking".
It is based on the AD dating system, (Anno Domine).
The turn numbering for Descriptive Table of Contents for TBFGK was referenced to the first page of the comic.
The battle of Warchalking happened on that turn. Thus 1 AW counts as the turn that the battle happened on.