shadowkreepM.2 is a form factor. With the exception of a few dirt cheap models that run it on a SATA interface, most M.2 drives run on a PCI-E interface.
shadowkreepThey're PCIe-interfaced. M.2 is simply a connection form factor; M.2 can run either on a SATA III interface or on a PCIe X4 interface. There are both SATA III and PCIe X4 SSDs that run on an M.2 slot. Those are NVMe SSDs running on an M.2 slot. M.2 is merely a form factor, not an interface type.