It may have 2x the areal density, but not necessarily the linear density. Imagine you have a string, that raps around itself in concentric circles, until you get a disk. This is how the drive is laid out. Now you can get double the capacity of the same area two ways. One is, to as you assumed, simply increase the amount of data per linear inch. That is, one inch of the string would be twice as much data. The other option is to wind the string tighter, so that there are twice as many turns, while leaving the linear data density the same.
Transfer rates will depend only on the linear data density, as the drive spins at a certain speed. So, it is possible that seagate left the linear density alone and simply increase areal density, which seems to be the case as they aren't anywhere near 80MB/sec.
(Sorry if I got any of this wrong, just peicing together stuff I've read from sites like this).