Initially copper, nickel, and bronze coins weren't covered by the NGC guarantee at all. The language was later changed in 2000 to add the 10 year guarantee so it is an improvement actually.
Yeah I encountered it by surprise. I had a red copper that needed reholdering anyway, but they explained that it does get reviewed and extended through that process.
They do change the language in their guarantee from time to time. If you don't check the guarantee from time to time you may find other surprises.
All in all, it seems pretty weaselly for a major TPG. Its like the used car dealers with the 50-50 warranty. If it breaks, they charge double and you pay for the whole thing. Probably motivated by a desire for repeat business. Get the people with older coppers to pay to protect their investment. Wherefore art thou, integrity?