They're a software and services company moving to a unified platform, active monthly users provides a better metric because it shows how many active customers you essentially have. Customers which browse your store, are more likely to follow products on your ecosystem and have subscriptions (XBL, Office365, Onedrive).
The guy is a developer, doesn't mean he knows exact reasons for the shift of metrics for MS PR. No offence to him. Although from Sataya's focus, it's bloody clear and just makes sense.
This is like saying a shop should count how many people walk into the store or look at their window because that's more important than how many items they sell. Ok leeh if that's what you want to believe then by all means.