I’m sorry that you can’t think of anything messy happening in bed other than eating food.
Scabs can also come off etc. Various ways to get bed linens stained. (I don't eat or drink--other than water--in bed. A cover just seems a reasonable precaution.)
Have you read the article? Children don't need to be eating in bed in order to soil sheets.
The commenter probably uses two sheets, one above and one below the body, so their uncovered comforter is protected from body soiling processes.
And not peeing on your hands is good too, but I still wash my hands after going to the bathroom.
Ah yes, "you're holding it wrong."
Don't shit where you eat. Don't eat where you sleep.