The amount written on the plaque is short by .5m
The comment we're all arguing about only says "50% shortfall" and does not say which side of the equation is short. So the word in that context merely means discrepency.
Maybe they did actually have the wrong idea about the story, but what they wrote does not say one way or the other, so there is nothing to correct and everyone is just picking a meaning and acting like they actually said more than they said.
English can't fabricate a missing identifier any more than any other language. There are no context rules that apply in this case to derive it indirectly, such as figuring out that "it" refers to something that was previously explicitly identified for instance, or anything like that.