Increasing the number of platters improves read/write throughput independent of the low-level technology used. As far as increasing density, that only scales relative to the square root of areal density. If you increase density by a factor of 100 you only increase the number of bits per track by a factor of 10.

The same rules don't apply to mram, of course, but from the article I'm not entirely sure if these techniques are useful for such designs.