So is it safe to assume in the next year that any games "starting" development at Nintendo do will be cross gen or Switch 2 exclusives? Not the ones they are already working on obviously.
Not really, depends how big the game is. They can start a new project now and finish it a while before the successor launches, or start a project now and not finish it till a couple of years after the successor.
They certainly may have projects in development that will launch on the Switch successor. BOTW development began in 2012, the same year the WiiU released and still ended up a cross-gen title.