Originally, Legends was an alternate universe
There's no evidence to this. From the get go DASH was established in a far future world that's flooded and filled with ruins because it has a history of endless war preceding it. The very title of "halcyon" days implicates that the prosperity and peace of the times is unique compared to that of the past. All of this goes along with the connection between quantum refractors and energen crystals, reavered and mechaniroids, Purge Official and Irregular Hunter, Irregulars and... Irregulars.
Really, the only basis for your claim is the use of "Rock" and "Roll" and other cute little throwbacks. DASH is very different from EXE, which from the very first moment was said to be in 200X with a setting in contradiction to the other story of 200X.