Agreed. The existing agreement means they can take it over from a military perspective. If he was simply saying that they shouldn't be expected to spend a fortune on their military whilst NATO partners don't bother, and be expected to bail them out when and if needed, it'd be hard to argue. If countries want equal protection under article 5, there shouldn't be disproportionate military spending.
Re minerals, Greenland have said they can fill their boots.
Fundamentally though, we and other NATO countries have placed over reliance on article 5, and essentially the US.