'Mixed-NOCs' doesn't represent any subtotal (contrary to EU GDP in your example), it is rather arbitrary mix of athletes from different countries.

The current medal table serves the sole purpose of NOC comparison - and 'Mixed-NOCs' introduces some confusion into this comparison. Look, it is definitely listed for comparison here, taking its place according to its medal count (even without rank assigned). Why not to show another medal table to the right of the current medal table? And this second table would contain medals of different NOCs, won by athletes within mixed-NOC teams. Mixed-NOC competitions shouldn't be considered somewhat less relevant, so this table is also a relevant one. It is separated just because there's no valid reason to combine them together within NOC medal table, since it would compromise the purpose of NOC comparison.