It literally started development right after Splatoon 2 ended, which is only a 2 year development cycle. It's going to be barebones as fuck, just like Splatoon 2//ARMS//Mario Tennis Aces//Super Mario Party were.
They'll fill it out over time but it won't be another year until it has an acceptable amount of content in it. Or they could just pull another Super Mario Party and not update it at all.
Nintendo are doing this so often lately that it's annoying. Breath of the Wild, Mario Odyssey, and Xenoblade 2 were so great because they were actual full games when they launched.