I don't see how it is guaranteed. I have a Z97 and Z87 chipset motherboard, both high end Asus. Only one will run NVME and only then at x2. From what I understand it is a limitation of the BIOS and chipset. Now I should preface that as running x2 as a boot drive. I was able to run it as a non-boot drive, but since I wasn't wanting to use it as a secondary drive I didn't check the speed.
Panteks Enthoo Elite / Asus x299 Rampage VI Extreme / Intel I9-7900X / Corsair Dominator RGB 3200MHz
MSI GTX 1080 TI / 2x Intel 900p / Samsung 970 Pro 512GB
Samsung 850 PRO 512GB / Western Digital Gold 8TB HD
Corsair AX 1200i / Corsair Platinum K95 / Asus Chakram
Acer XB321HK 4k, IPS, G-sync Monitor / Water Cooled / Asus G571JT Laptop