Maybe VW had a massive box of bolts to use up? That does seem excessive to hold a heat shield on. Do the 18mm bolts hold anything more substantial else in place as well? I haven't looked under there yet.
Funny - I was thinking the same;
"Hans - we have a a lot of these really big head, dumpy short length of thread and a beefy captive washer bolts - what can we do with them?"
"Franz - use them in something ridiculously light that is self supporting - it will confuse people in 15 years time!"
These are my left over spare bolts from the head re-build (I can't find a home for the other black bolt??) - the 2 big ones are the ones in question:
They live above the exhaust ports, one between ports of 1 & 2 and other between 3 & 4
-They don't hold anything else except the heat shield and then in turn the coolant line.