Putting effort into Windows on ARM development made some sense when Windows had mobile phones. Without Windows Mobile, ARM devices don't have enough penetration in the platform to make a difference.
I suspect Microsoft is going to be feeling the impact of abandoning mobile on their other projects and goals for many years to come.
The author suggests that Microsoft try to build a developer ecosystem on ARM, forgetting that Microsoft already spent a ton of money on developer evangelism and sponsoring ports of apps to Windows on ARM for Windows Phone and the early ARM based Surface/Surface 2 to no avail. It really makes no sense for them to continue to flush money down the drain.
The difference is that the Surface Pro X is running true, full Windows 10 ported to ARM64, not Windows Mobile or that hobbled OS that ran on the earlier Surface tablet. It's a different device entirely.
This is probably the biggest challenge for ARM and its partners today. It'll be interesting to find out how they incentivise developers and envision the enable ecosystem efforts in this direction.