Easy to prove. Fill the mark using the procedure described in the prior post.
Drive for 45 minutes at highway speeds. Pull over and check the level. It will be too high, due to heat caused fluid expansion.

That's the reason for filling to 10mm below when cold.

It's obvious that not all mechanics are rocket scientists, or for that matter that all people who claim to be rocket scientists are rocket scientists.
You have MB shop's using the wrong anti-freeze etc. etc. So you pick the advice that makes the most logical sense, follow the MB manuals and go for it.

Steve