Nothing's wrong with the statement, it just depends on the build. If you build purely tanky with lots of resists, your W>R>Q>E nova will do all of, like, 500 damage after resistances. Which is less than I'll do in one swing with Nasus' big stick (not really, unless I crit, in which case yes really).
To make Maokai's nova highly damaging you need to go AP. Which is alright, but why would you do that? When you have Kassadin or Malzahar or Annie to choose from?
Basically, the way I approach this is as a player with 20+ heroes in my roster, who do I fling out into combat for me? Do I pick the guy who is not as good for the job but who I think is really cool (Maokai) or the one who is really effective at his job and still pretty fun (Nasus)? It's not that Maokai is so bad he can't possibly be used.
But he's actually a worse tank than nearly any other character in my roster. Let's look at Kassadin. His base resists are lower than Kassadin's, so Kass gets the win for defensiveness. Sad but true, Tree! In fact, aside from Maokai's ult, nothing about the tree is good for tanking. Yes, I do love his W soooo much, but I have to admit that Malphite and Amumu and Rammus and even Shen have similar kinds of abilities that are better. I'd rather have a 2 second AoE taunt on Shen than a 2 second single target Root. Even his ult is somewhat debatable. Maokai is certainly not a 'tank' to me, but a support caster like Soraka. A damage reducer ult, some bonus damage attacks, and a ranged support ability.
So I'd never build him as a tank, but I'd hesitate to field him at all if my team needed some AP burst damage. He doesn't do any role well enough to beat out the competition. He can't tank better than Shen, Rammus, Malphite, Amumu, Udyr, Nasus... etc. And he's no Annie, Kassadin, Malzahar, Morgana... etc. So what's he do that makes him worth picking in a serious game? I think that's what he needs.
I plan to continue playing him, I just don't want to lie to my team and say it's the best choice I could make.