My only guess is maybe the cloth picked up some skin oil from when I wiped my smartphone screen with it. Other than that, it was clean, nothing on it. I don't use those cloths on anything else.
Existing contamination of the cloth, laundry additives/softener, being around cooking, fingerprints around the edge of the screen, the screen touching keys with skin oils on them...
This is why when cleaning optics professionally, they always use disposable tissues or wipers. Fingerprints can be dissolved, but worse is if you get sand or similar particles on the cloth, that's permanent damage.
Even after using a separate cloth and some water, I couldn't get smudges/smears off the screen. It got to a point there I was probably rubbing it too much to get it clean.
Rubbing is how you ruin the display coating. That's far worse than using an appropriate solvent.