I didn’t mention any purchase of some sort, only the release of unreleased DLC content for the Remastered Edition of Cities Skylines for consoles.
However, I bought the game as digital version on Xbox in 2017 and also some DLCs in bundles for a discount price.
Since I own all DLCs for the Xbox version of CS, I could download all these contents again for the remaster FOR FREE.
At least on Xbox, it’s possible to purchase DLC for the Remaster that can also be used in the old Xbox version as well. So it works both ways, for owners of the old version, as well as for new players that start with the remaster and the Xbox One base version.
As far as I know, physical disc and PS+ versions are excluded from this deal, which is a pity, but reality.