He said this on the Game and Talk podcast here:
"Nintendo WILL announce the Switch 2 in March. Everything I'm hearing dating back to Gamescom last year has indicated something was happening in March. That talk has resurfaced in the past few weeks, and it is indicating that the Switch 2 is set for a reveal or an announcement in March."
He also mentions his Partner Direct talk as "informed speculation" as well as the idea of no Joy-Cons again. That said, Nate doesn't know what form the reveal will take.