I read the whole article, and from what you have written i hope you meant about this part.
"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."
If so it means you can not use an SSD meant for "Mid-2012" and "Early 2013" for a laptop of "Late 2013" and "Mid-2014" or vice-versa, they are still removable and replaceable but please do make sure that the replacement is intended for the model that you have.
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