Can Wi-Fi read through walls?
Yes, but only if the source and the detector are on opposite sides of the object being 'read' (just like a medical X-ray really).
So yes it's possible under limited circumstances, but the 'letters' and the 'reading' are pure red herrings (unless you have alphabet wallpaper with very special high frequency characteristics).
I'm getting increasingly worried by the extent to which science is increasingly getting reported by its researchers using journalistic click bait. Science used be about objectivity - now it often just seems to be about getting noticed -- another example of "look at me, mum!" in a world of shouting children. It's a pity, bacause the sctual science often merits more adult attention.