Diomeia (the more authentic form of the name) was one of the Attic demes - actually an urban quarter of the city of Athens itself; and many sources mention the Herakles-shrine (Herakleion) there.
The present entry abridges the rather longer one in Harpocration s.v. (commenting on Hyperides fr.114 Jensen), and introduces confusion in doing so. The festival is not a gloss for the shrine; Harpocration had mentioned both (saying that the festival features in 'the comic poets').