If they were to release the DLC before the Switch version came out, there would have probably been an uproar. I wonder if they have been working on the DLC simultaneously to the Switch version, meaning we may see the DLC shortly after the Switch version releases in a couple of weeks. They were very quiet about the Switch version, and then suddenly we were given a release date just around the corner -- the same sudden surprise may come with the DLC.
I think the DLC may come by/around April, one year after the game's original release date. I don't think we will be waiting into 2019; if we are, I'd be expecting a huge update, which I don't think is the intention.