ND definitely confirmed it, just not how consistent the rate is.
I really wouldn't worry about that.
Actually I don't understand if you're ''worried'' that the frame rate will be 30fps with spikes to 60fps in the most barren, calm sequences?
Naughty Dog would never claim 60fps if that was the case.
As with every other game 60 fps means that the game in ''normal situations'' will always be at 60fps.
Of course if the frame rate drops with many enemies on screen and tons of effects going on, that's pretty normal. If we're excluding from the 60fps list, games with frame drops in the most intense scenes then probably that list will be really really short.
Personally I was pretty satisfied with the fluidity of the Tomb Raider Remaster and that was more 45fps+ compared to 60fps.