You have a point. But those Greek Euro coins and notes eventually flow through the banks. They could be removed from the money supply at that point.
The big point here is that if Greece leaves the monetary union and all of the Euros they printed and minted stay in the money supply then their effect on the Euro will still be there. Remove the traces of their coinage and printed versions of the Euro and you increase the value of the remaining currency as there are fewer coins and bills.
It’s a type of deflation if you will.
They would have to remove them as they turn up in other countries (as they do with the US $1,000 bill, and are starting to do with the Canadian penny).