Something is either historic, or it is not, there is no graduation.
The name of the programme is a bit clunky (which fits with the content, based on the episode I half watched), but I don't agree with this bit.
'Historic' means more than just 'happened in the past' - because after all, not every past event is deemed worthy of being recorded in the history books. There's an element of fame (or infamy) and importance implied.
Even without that distinction, is there not also a chronological element? An event that happened last week might be considered less historic than one that happened a thousand years ago. Given that the series is looking at towns where momentous events occurred in the past, I think it's much more acceptable in this case than the normal everyday lazy hack usage you see so often, where any vaguely important
current event is described as 'historic'.