Liability release. If you pull right back in and say “ever since you changed the oil, my engine started knocking”What was the point of them telling you that ? They don't do engine repairs or replacements.
My assumption(from a guy who was not there nor has ever looked at the car so not worth much)was that this was either a miscommunication or mistake on the part of the guy pulling the dipstick. Maybe it was really clean oil still and he thought it empty. Maybe the car in the bay next to yours had no oil on the stick. I don’t see how it would be to their advantage to lie about something of that nature.