By the argument of your first paragraph, Android and iOS are no threat to Windows because they don't run Windows apps.
But of course they are, just as Samsung is a threat to Apple, Apple is a threat to Samsung, etc. We are doing a set of tasks and using features and functions that multiple options provide a solution for.
Platform tie-in was once a strong bond, but has dramatically weakened. Those people who chose teams in the nascent days might be somewhat locked in, but anyone picking a platform over the past half decade has almost always created a transferable world.
As a case in point, my wife recently went from a Nexus 5 to an iPhone. Virtually nothing in her life changed: She still had gmail and the KFC app and Facebook and Hangouts and a good, standards compliant web browser and SMS, etc. For a variety of reasons she decided she didn't like idioms of the OS so she moved to a Galaxy S8, and again her world was close to identical.