If the "native" Windows app it's running on a dedicated browser and coded with web tech, it's still a web app even if it's an .exe

Did you know that Picasa could still group people by facial recognition in windows XP with 1 gb of ram in TOTAL? (Clarifying: 1gb to the whole system, not only a single app)

And are you sure that the indexing is running locally? Because it's called "onedrive photos", not "windows photos", so they can totally do the indexing on the cloud.