Nintendo has no established brand presence or background of marketing sway in the health/wellness market, I don't think they would bet on 20 million orders for that. Beyond that Digitimes knows their consoles pretty well, I don't think they'd get a game console confused for a sleep sensor or something similar.
20 million was however at one time basically their standard yearly shipment for GBA + GameCube. So if NX is a line of a game hardware (both portable and home), then that part makes sense.
Even with the 3DS + Wii U, the first year 3DS shipment was just under 14 million in 12 months or so? Wii U was about 4-5 million in its first 12 months I think. So there you go -- that's about 20 million, so you can see why Nintendo might have settled on that number.
