attach rate gets used two different ways. i think you're thinking the reverse of the other posters.
What does "highest attach rate of software and accessories" mean? That people are buying a Switch as well as a game at a higher rate than they bought a PS4, for example, and a game? I mean... wouldn't that be obvious when playing Zelda is the only thing the Switch does..? What am I missing? Not hating on N's success. Just seems like a "well duh" kinda comment.
if we're talking "The attach rate of zelda" then it's a percentage like, "XX% of switch buyers bought the game," to which the reasonable response is "holy hell did some people seriously buy this thing for 1-2 switch or bomberman?!" and of course that's a really high percentage, closer to 1:1 than most launch titles.
but it also gets used to mean "how much crap did each switch owner buy on average?" so like if the average shopper bought 3 games that's an attach rate of 3. in which case, we're all surprised if this is a true story and not just marketing speak cause we expected the majority of people to just buy zelda.
a few million lapsed zelda fans who'd rather own this than a wiiu can provide strong sales for a bit.
At this point, the only thing I can think of is that the machine's visual design was great and the ads worked well.
I cannot think of any other reason as to the Switch being Wii~levels of success.
but yes the hardware itself is slick enough that I can imagine people i know who normally aren't nintendo fans buying one just because it's such a tempting toy.