The article is from December 1987, when nobody yet knew that it would end up that way. The Compaq Deskpro 386 had just been released in 1986 (thus unmooring the “IBM PC clones” from IBM), the September 1987 release of Windows/386 2.01 was only a couple of months ago (less if you account for print turnaround), and development of what would initially be called NT OS/2 would only start in 1988, with the first documents in the NT Design Workbook dated 1989. Even OS/2 1.1, the first GUI version, would only come out in October 1988 (on one hand, really late; on the other, how the hell did they release things so fast then?..).

While NT OS/2 effort started earlier, Windows 3.0 was apparently an unsanctioned originally rogue effort started by one developer, initially masquerading as update to "Windows-as-Embedded-Runtime" that multiple graphical products were shipping with, not just Microsoft's

Even when marketing people etc. got enthused enough that the project got official support and release, it was not expected to be such a hit of a release early on and expectation was that OS/2 effort would continue, if perhaps with a different kernel.

Microsoft was only interested in fulfilling the contracts, and some networking components such as NetBIOS and LAN Manager, then winding down. This was due to Microsoft had already been in discussion with David Cutler, and had hired him in October 1998 to essentially port VMS to Windows NT. Windows NT 3.1 appeared in July 1993.

https://archive.org/details/showstopperbreak00zach

Microsoft unwrote a lot of the code that IBM needlessly wrote.

I worked as a trainer at a commercial training company that used the Glockenspiel C++ compiler that required OS/2. It made me sad. NT made me happy.

This is from 1987, the IBM / Microsoft joint development agreement for OS/2 didn't fall apart until around 1990, and there was a lot of Microsoft work in early OS/2 (and conversely, non-multitasking MS-DOS 4.0 was largely IBM work).

Windows NT originally shipped with an OS/2 compatibility layer, along with POSIX and Win32.

I'm assuming that all of it was written mainly, if not solely, by Microsoft.

If you count the beginning as the time between OS/2 1.0 up until MS released Windows 3, then it makes sense. IBM understood Microsoft would continue to collaborate on OS/2 more or less forever.