That’s why it would need to be a small piece of a greater set of information. Imagine a person walking through an office and into a stairwell. If you know the person is there, and you know the stairwell is darker than the office, you could infer the appropriate location of the person in the building

That has nothing to do with the technique described in the article. It's also still quite a stretch. Holding up a piece of paper and casting a shadow on the ambient light sensor will also make it appear darker. Are they in the stairwell or is Bob from accounting stopping by to tell a "funny" anecdote and blocking the afternoon sun? If you've managed to compromise a device enough to access sensor data, you're not bothering trying to make sketchy assumptions based on the light sensor.