I'd disagree with that. I think that generally by level 6, a decent Nasus can become pretty much untouchable in lane. Especially if the opponent is foolish enough to push against him, as Nasus thrives when farming under the tower. And Nasus IS one of the few champions who quickly becomes strong enough to dominate 1 vs 2. I'd say as Nasus, more often than not, when I'm ganked I choose to stay and fight rather than run, and it typically works out well. Particularly after you have your ultimate. The best way to counter him is to pick a strong duelist and aggressively zone him without pushing.
Had a game last night against Renekton (a counter to Nasus, though not the strongest). The guy goes in very aggressively, dives me under the tower at level 2, and dies to tower damage. From that point on there was pretty much nothing he could do to me in lane. Anytime he came too close, I'd give him a Q, then go back to farming. Ended up carrying the game, though it wasn't easy despite me being ridiculously fed (my Q farm was a little low though, due to every time I got a second to sit down and farm a wave, one of my teammates would initiate some terrible teamfight and I'd have to run over or teleport in to try save them). Teammates were flaming each other and trolling hard, and the enemy Lucian did a good job of kiting me with his infinite dashes and Blitzcrank knockup. Our Evelynn would literally run into the enemy 1 vs 5 when none of us were anywhere near her, die, and then yell 'Where were you guys!!??'. Crazy.
But yeah, Nasus is strong. Of all the champions who I'd say have the potential to carry a game hard on top lane, I'd say the best are him, Ryze, Darius, and Jax. Renekton is strong, as DemonOfWrath said, but generally I don't think he can carry a game alone.
Nasus does have a pretty good level 6 but so do a lot of other champs top lane, and typically you'll be shoved in and harassed before then so you can't start going aggressive then because you'll be low of hp and/or mana (oh and if you get ganked early you're kinda fucked because you have no way to escape). Nasus hits his stride at level 9 because that's when your Q goes to it's lowest cooldown and you can just bop the other guy repeatedly, also it's about when you finish your first item so you'll have the tank stats to go with the bopping. Of course I'm assuming your opponent is playing it properly as well, like your example of a Renekton tower diving you level 2 is what? And yeah after that if you didn't crush him I'd be surprised, the lead you would've gotten was probably ridiculous.
Aside: Renek actually has a mediocre lv 1, strong lv 2, and then becomes a powerhouse lv 3 and spikes hard at 6. If you're tower diving someone as Renek before lv 3 it's either an instant kill then jump out or you're being dumb. Most people just assume Renek is going to crush them immediately, when a lot of things can beat him lv 1 and set the pace of the lane to be completely different than you'd expect.
I also disagree on Darius being a hard carry from top. He's basically a different Renekton (scarier up close and in drawn out skirmishes, but Renek can actually jump on you from a distance) but utterly terrible at teamfights. If I'd put someone with Ryze/Jax/Nasus for your traditional hard carries from top I'd be tempted to say Irelia but I think those three are alone there. But I stand with my previous point of top lane carrying being more about map presence and objective control than getting strong enough to 1v5, which is why I like Renek because he has mobility, tankiness, damage (with tiamat or brut) and CC so he's awesome at setting things up for your team if you play the map well.
Don't get me wrong, I love playing Nasus myself but my experience says it's too team reliant because it takes so long before you can affect the rest of the map strongly compared to other stuff. Too many games where I've farmed well, just to see that mid and bot have fallen flat enough that I can't turn it around because the enemy is fed enough to explode me as I try to walk in. I just find it's more effective to pick something that's capable of doing stuff to turn it around before it gets that out of control.