[imeron]

Just for the older Retina models. Copied from the link you provided: "Specifically, the "Mid-2012" and "Early 2013" models use a 6 Gb/s SATA-based SSD whereas the "Late 2013" and "Mid-2014" models use a PCIe 2.0-based SSD. These SSD modules are neither interchangeable nor backwards compatible."

[MiguelHudnandez]

The reason for the sentence you quote is to explain that PCIe chips are different than the proprietary-pinout m-SATA port in 2012-2013 Retina MacBooks. They are strikingly similar and if you're not paying attention, you might try to move one into an incompatible machine.

You can still swap a drive in newer models, just with a PCIe card instead of an mSATA drive.

Simply put: Newer generation of hardware = newer storage interface.

Here is a replacement guide for a 2014 model Retina MacBook Pro:

https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Disp...