Your post is making the mistake that, just because all this function needs is detection, that therefore this is the only thing it's actually doing. "What this is about is..."

This isn't about detection and isn't about triggering a sleep mode at all. It's not even about over-engineering for this particular function, because this particular function is not the point. What it's actually about is doing everything they can conceivably think of to monetize engagement tracking, while continuously pointing a camera at your face as you watch movies and adds.