DEC eventually shut this down, which prompted his departure for Microsoft. This is unfortunate for DEC, as they eventually poured the company into their Alpha RISC processor, which did not live as long as DEC hoped. Prism might have been a superior design.
At this time, Microsoft was maintaining a UNIX kernel in their Xenix product, so they knew a good kernel engineer when they met one. Microsoft was the leading UNIX vendor in the early 80's.
Cutler famously disparaged the UNIX kernel (his notable saying was "Get a byte, get a byte, get a byte byte byte" to the tune of the finale of Rossini's William Tell Overture).
Microsoft dumped their Xenix onto SCO about this time.
What is more interesting to me was Cutler's involvement with Azure. He must have had some sway over CBL-Mariner, Microsoft's RPM-based Linux distribution.
Much of Cutler's earlier work is documented in the "Showstoppers" book:
The book doesn't really delve into the Xenix decisions, if I remember correctly.
Without Cutler, Microsoft would likely have ended up on a BSD kernel, as Apple did.
From your source: "Microsoft's internal project name for the new OS was OS/2 NT, because Microsoft's intention was for the new OS to succeed OS/2 yet retain the OS/2 API as
its primary interface."
Note how Russinovich talks about a "new OS".
The NT (kernel) never had any OS/2 code in it, it was a totally different project. The first target processor architecture for NT wasn't even x86, it was MIPS.
OS/2 in NT was merely a subsystem layer next to DOS and Posix, somewhat like today there is the Linux subsystem layer in Windows 10.
A good source is this:
It was published in 1994 (so covers up to Windows NT 3.5 and touches on Chicago/Win95) and features a lot of info gleaned from 1:1s the author had with Gates, Cutler and a host of other executives.
(The design decisions made back then between 89-93 for NT are what enabled things like 'Bash on Ubuntu on Windows' to take place today.)
Digital dropped the ball in the late 80s with regards to management of Cutler and his team, canceling his PRISM project and leaving him and his team disgruntled.
Elsewhere in Seattle, a chap named Bill Gates was flush with billions of cash and knew that the shelf life of DOS was limited; if Microsoft were to succeed, they needed a new, robust, reliable and high-performance OS that they could "bet the company on".
Gates got word that Cutler was disgruntled at Digital, and a mutual party set up a meeting. Cutler was dismissive of Microsoft's technology stack at the time (DOS and some office apps) -- he was a hardcore OS engineer, and DOS was a toy.
Gates persisted, ensuring Cutler that he would have the opportunity to build the next generation of OS from the ground up and essentially unlimited resources at his disposal to do it. Cutler eventually agreed, and the NT kernel project was born.
'Gates', Stephen Manes 
'Show Stopper!: The Breakneck Race to Create Windows NT and the Next Generation at Microsoft', G.P. Zachary 
'Microsoft in the Mirror', Karin Carter 
But I agree with the parent that 'Programmers At Work' is a great book.