In their description:
This Mac Pro is fitted with a brand new 250GB (NVMe PCIe Solid State Drive) with the Apple Mac OS High Sierra System installed. (They never say High Sierra is actually installed on the NVMe. If I read this sentense correctly, it means "This Mac Pro fitted with a NVME SSD", and "this Mac Pro with HS installed". But not necessary "the NVMe with HS installed)
Until now the faster more responsive NVMe Solid State Drives have not been bootable on the Mac Pro workstations, it offers six time the performance over a 2.5″ SSD installed in the regular SATA bays. (And they also said NVMe is NOT bootable on that Mac Pro. May be there is another SATA SSD in one of the HDD bay for the OS)
What do you think they mean by "until now"? That implies they've made a change.
I have no idea about whether they have installed the 140.0.0.0.0 firmware or have modified a previous firmware for NVMe booting or neither. I was simply responding to the other poster who stated that "They're already selling bootable NVMe-equipped MP5,1s..."
If they're doing that, then I think it's highly irresponsible. If they aren't then it's not.