I think Nintendo's being coy on purpose, not that they don't actually know. At this point with modern development being as complex as it is, the game is either done and able to ship in the next 60 days, or it's not. They'd know by now. I think they are positioned to do some "surprise! These are waaaay sooner than you guys think"-type announcement timing, specifically in regards to Metroid and Kirby Air. That's my sense. But they also need to hold off til closer to Gamescom or the usual big September Direct.
Sales significantly spike in September following their Directs, as time has gone on. And people expect it. And it's not like we need a lot of notice. You're either going to make time for it and buy it, or you're not.
So I'm 50/50 on Metroid being either October or November at this point. It'll do well in either month. Will certainly be the highest-selling title in the series, building on the success of Dread.