Lot of overlap in that 250m figure.
Given the undeveloped state of mobile gaming at the peak of the DS, Wii households typically had at least one DS. The DS itself became a collectible with all the special editions, further padding that install base figure.
The prospect of the Switch taking back market share from mobile in addition to whatever is left of the dedicated handheld market (zero growth/eroding niche) alone would make it a success given the state of Nintendo's console market following the Wii U.
So sure; by the numbers, even if the Switch squeaks past the 80m mark, it means they will have only sold the same number of hardware units as their previous generation, but the reality is that the back to back hits of the DS and Wii will in all likelihood never happen again in the console/handheld market, no matter whose name is on the product.