i feel like nsmbu dx was created when certain things fell out of the fiscal year plan - namely, yoshi and fire emblem. yoshi i could still see happening in march, but fire emblem is probably may or later.
I knew about NSMBUdx as early as late 2017, so that's very unlikely to be true.
What's more likely is that something else was supposed to release in Jan-March that has now been internally delayed (Probably Yoshi's Crafted World - it was in a very rough state during E3 2017) and Nintendo felt the need to fill the gap by pushing back NSMBUdx. Basically the DKCTF Wii U strategy.
That being said, you're right about 2019. That is basically going to be the Switch's equivilent to the 3DS' 2013. The year where the big guns are finally all ready and the system explodes! It's gonna be incredible! (Hell! The lineup is already reminiscient of it! Let alone what else gets announced!)