Yoshi and the Mysterious Book will sell for $60 in the eShop and will have an MSRP of $70 for the physical cartridge.
An opmist would say that it is $70 game and they are adding a $10 discount for buying it digitally. A pessimist would say that it is a $60 game and there is a $10 fee for the plastic cartridge.
I imagine they’re using the eMMC data bus standard, but it’s pants-on-head moronic to use flash chips for what’s supposed to be permanently nonvolatile storage. Flash media is not storage stable if it’s not powered regularly; ask anybody with an old Windows install USB drive that’s more than a couple of years old and they’ll tell you all about it.
If this is so, that also opens up the inevitability of the data in cartridge games decaying and eventually becoming lost forever. Your cartridges would literally rot on the shelf.
Actually come to think of it, I wouldn’t put it past Nintendo to do that deliberately and be salivating at the very thought.