for the non-pedologists:
Sandy clay is a textural box on the soil triangle. It’s small and requires the absence of silt. The reason we joke about them not existing is because of the size of the box and how far apart the two grain sizes are. There are very few scenarios where sand and clay would be deposited and not silt.
Sandy Clay has ruined many an embarkation location for me in Dwarf Fortress because the map will say it has sand and clay, but sandy clay is not able to be used for sand or clay. I don’t even think you can grow crops on it.