With thanks to AI…
”Angel Delight doesn't set with oat milk because its powdered mix relies heavily on specific dairy proteins (like casein) and fats to thicken. Oat milk lacks these proteins and has different fat and sugar structures, which typically leaves the mixture watery or soupy.”
This must be why the packet says that whole or semi-skimmed milk should be used.
What was his post about Angel Delight? I searched but couldn’t find it (maybe a blessing in disguise?!)