He pairs really well with ranged carries, but that doesn't make him any less of a hard carry or monster dps. Two carry strat is popular for pretty much any team composition(with exceptions of course), regardless of n'aix. He has a steroid (with a bkb), an insane lifesteal/damage passive in one ability (DOESN'T EVEN TAKE UP THE ORB SPOT), and a modifier on that lifesteal (which in turn modifies his damage). That's pretty much the definition of a hard carry.
It's like saying TI2 morphling wasn't a good hard carry just because he wrecked everyone in the early + midgame and won by 40 mins. N'aix is very strong early + mid game (which is why he is "in" and one of the best trilane heroes), but that doesn't make him any less relevant at 60 minutes (which games rarely go to anyway).