That's the suggested PPI the display would have... if they were to make an 12,9 inch. However I still have my doubts and I think they will just increase the PPI of the original iPad Air 2.
PPI doesn't change radically in order to facilitate integer scaling of apps and manufacturing yields of displays. If/when Apple starts requesting uncommon display resolutions/densities, it ends up costing more until the product sells well.
The iPhone 6 Plus solution of hardware scaling is less than ideal, you are using power to do that. The number of companies that were already building 5.5" 1080p displays such that Apple could ask for a "better" one, rather than something completely knew is why it's less feasible with the iPad Pro.