I've just tried it on my large desktop monitor (roughly 1440p, not HiDPI), and I now see "Ghost Font" extremely clearly and can't see the decoy at all. If I scale my browser window to 30% zoom, then I can just see the "Written In Ghost Text" decoy message again.

My phone would have been zooming out the browser window, and making the dots even tinier, but the phone is HiDPI so it would have still preserved the dots. My eyes are middle-aged and probably starting to do the same kind of median-blur effect that models do when they resize an image. That's my current guess for why I can see the decoy more clearly on mobile.

If that's the case, then this trick will stop working as vision models approach pixel-perfect vision, instead of the current resizing that they do. Pretty cool as steganography though.