It's pretty easy to disagree with this. There are shareholders that are still upset at the hundreds of millions spent on marketing and strategy in mobile.
The Microsoft that was worried that ARM was the present/future and Windows needed to be front and center on it, had Windows Phone as a major cross-cutting concern touching every single piece of that balance sheet:
Microsoft made a big play for enterprise licensing of Windows Phone. The last Windows Phone 10 handsets on the market were enterprise sales only devices from vendors such as HP.
Microsoft saw Windows Phone first and foremost as Windows. When Microsoft made that commitment to have Windows 10 running on a billion devices in a short time frame, a lot of their hubris in setting that expectation of so many devices so fast was that they saw a huge growth in ARM devices, especially Windows Phone that never came.
Windows Phone had some of the first attempts at Xbox mobile efforts. The death of a lot of the entertainment division, specifically Microsoft's deeper plays into Music, eBooks, TV/Movies is often directly blamed on the death of Windows Phone leaving Apple too far ahead in pole position on those services.
Windows Phone had big plays from nearly every Service on Microsoft's books. Cortana was built for Windows Phone and the death of Cortana as a front-and-center Services play, too seems to rest on the Windows Phone's shoulder.
Windows Phone was also directly the driver of what most shareholders think to be Microsoft's greatest (ever), most expensive blunder in M&A: the Nokia acquisition.
I think it contrasts directly with the commitment Apple made to the iPhone; had the iPhone failed Apple would have been left with as many scars as Microsoft still seems to have. More than half the Entertainment Division doesn't exist today and Windows Phone is seen as a key reason. The Services team has seen almost exactly as much turnover, again due almost directly to Windows Phone scars. Almost none of the divisions exist in the same form they did today as they did when Windows Phone was the big existential play for the company, the big bet that failed. (If that doesn't show commitment, I don't know what shows commitment.)
It even left scars such that Apple, following in Microsoft's footsteps to ARM on laptop/desktop, years later, seems to get direct credit for things that were Microsoft's efforts. To the very point of the article here and other threads in this discussion, Electron on Apple Silicon would not have happened nearly as fast if it hadn't been for Microsoft's ARM focus/efforts directly forcing Chromium (via the Edge team) to support ARM. (Edge probably wouldn't have switched to Chromium as its renderer if Windows Phone hadn't failed, and if it hadn't needed to spend so much directly in the Chromium codebase in the effort of forcing Chromium to adopt open source ARM builds.)