M2 is a standard that supports both SATA and the awesome NVMe. If they are saying that the M2 is running through the chipset, then it is probably SATA based. If they are saying it's to the CPU then it's the newer, better future PCIe NVMe variety.
As for these big sequential or bandwidth numbers you see, it's best to understand those as MAXIMUM speeds. On SATA if you want 100MB of data, you will get those at roughly the same time whether you are on SATA II (300MB/s) or SATA III (600MB/s). However if you wanted 301MB of data you have a greater chance of getting it at half the time.
The story is different with NVMe primarily because of latency. The SATA controller is no longer there. You remove one chip from the data path. Of course that's assuming performance is the same or better on the NVMe SSD controller... if the NVMe SSD controller has more latency than it's SATA SSD version, then they just shot themselves in the foot.
