OK, so I've taken a hard look inside and... Viola! There is indeed some sort of chemical pack inside. So apparently, the particular MB tech I spoke with originally was unaware.I did replace my coolant tank with new MB and it did have a chem pack in it. Check with FCP euro.
So I called the dealership back and talked to the service manager. He was aware of the chem pack and told me that what it does is provide a sacrificial place to oxidation to occur instead of in the engine/radiator. He said it is not for extending the life of the coolant, but just for internal chemical reaction protection. HE also said that is does give some protection if people use tap water instead of distilled. And yes, it should be replaced when the fluid is changed, assuming the fluid made it a normal service interval. I still can't find it in the STAR service manual, which is disappointing.
I'll still stick to my guns and say there is no way coolant should get pushed to 15 years unless the vehicle is not driven very much. I get to 150K miles in 4ish years, so that pretty much meets the 5 years service life for G-05 per Prestone's specs. And if the fluid manufacturer says 5 years, then its 5 years, regardless of miles.
So, now I know for sure.