You're also overlooking the fact that they did some relatively interesting things to notice when someone posts a PNG-of-something-that-could-be-a-JPEG, and can then use a lossy image instead of the larger PNG.
Read the article. Short answer: no, while your image content breakdowns are accurate it's not actually that simple in practice as things like screenshots are pngs.
They've also got to handle photos with borders, screenshots, and such, which may or may not work better as PNGs. And anyhow, why wouldn't they want to handle un-photo-like photos and un-logo-like logos optimally too?
