I am very concerned about your dealer telling you such a thing. There are marks on the crankshaft damper and marks for the timing chain pully. It's a relatively simple operation to remove the valve cover and turn the engine to top dead center and see where the timing chain sprocket aligns. If it does not align properly the chain is stretched.
Unless one of the pro techs responds that there are no marks on your particular model (in which case I will be VERY surprised) they should be able to do this without too much trouble.
I hope their not trying to take advantage of you.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in