There is no technical reason that you couldn't run NT on anything you could run Unix on. It was architected from day 1 with a hardware abstraction layer (HAL) specifically for that. In theory, port the HAL (assembly language) then the rest is just a recompile. There's more to it than that (the SPARC version suffered severe performance issues due to endianness) but that's the general idea.
I expect Dave Cutler remembered that VMS (at the time) could only run on VAX, the OS and CPU were developed in tandem and VMS relied on some features not present on other CPUs (I don't recall exactly what offhand). It was a huge engineering effort to get it onto Itanic.
The main problem was that a single Alpha could replace an awful lot of VAX HW and so to claw the profit back, the license fees meant a VMS Alpha was about 3x the price of the same machine running NT.
We used to buy NT Alphas and install Linux - to compete with Sun Sparcs.
I expect Dave Cutler remembered that VMS (at the time) could only run on VAX, the OS and CPU were developed in tandem and VMS relied on some features not present on other CPUs (I don't recall exactly what offhand). It was a huge engineering effort to get it onto Itanic.