That would be correct - most LED tail light assemblies are not serviceable and you must replace the entire fixture. Should be covered under warranty though so just show the dealer and let him order a replacement. Those lights are not cheap so push hard for the warranty.
When the new unit comes in, a good dealer should do the swap while you wait - it's an easy job.
BTW, I doubt your wash job damaged the light. It was prolly bad and you did not notice until later. They break right away or they last a long time.