Why are we saying that they're Heartless? Like members have said, they're called Black Costumes and they don't seem to have a Heartless symbol, only what seems to be a crimson Recusant Sigil. Other than that, Pureblood Heartless don't have colored appearances. They're just dark with bright yellow eyes.
By the way, this game definitely has to take place in the Sleeping Worlds. There are Emblem Heartless everywhere in chi/Unchained, and assuming this game takes place in the distant past before the Keyblade War, then it's impossible for Emblem Heartless to have existed back then.
Granted, it wouldn't be the first retcon in the series, but I doubt this is a retcon, but more like an indirect hint about where chi/unchained really take place.
By the way, this game definitely has to take place in the Sleeping Worlds. There are Emblem Heartless everywhere in chi/Unchained, and assuming this game takes place in the distant past before the Keyblade War, then it's impossible for Emblem Heartless to have existed back then.
Granted, it wouldn't be the first retcon in the series, but I doubt this is a retcon, but more like an indirect hint about where chi/unchained really take place.