As you say, it's all about supply. Will they make that many systems?
Given that they're still selling 400k plus a week in August, and must be stocking up for the holiday by now, I've gotta think they are producing a heck of a lot more Switch's than the 20 million or so that they have official forecast.
Given that last year they sold almost 11 million in the holiday quarter, I could absolutely see them selling 13+ million this holiday quarter if they have that many available. I think to hit your Q4 number though they might need a big early year release as it'll have been almost a year since AC launched. But I could see 3D World port and BotW2 or at least one of those hitting early next year, assuming of course BotW2 isn't on the docket for the holidays.
So to me it would come down to are they actually making/shipping 30 million Switches this fiscal year, and do they have a big game launch for Jan/Feb to push sales the quarter after a record breaking Christmas which will presumably meet demand for the holidays if they have 13+ million sold this Fall.