"Always comfortable" might be a better choice of words. True happiness goes much deeper than giggling once in a while. That's extroversion + childishness. "Happy" means that, overall, you're satisfied with your life and the choices you made and that you feel you've gone as far as you possibly can or need to.
But it's really fucking stupid to pidgeonhole this type into a polyanna role. Blegh. No one behaves like that. If anything, that's an idealization from an FJ point of view, highlighting all this type's good traits in its prime while ignoring the hellscapes of impulsivity that characterize the rest of its life.
From his experience, this guy says that ENFPs are often exceptionally aggressive as a form of compensation.
Ne-doms are known for their lethargy and tendencies to be distracted. That doesn't exactly put a person in his prime. And Fi doesn't make you automatically contented either. Fi/Te is like a flame trying to pierce into the cold darkness, Fe/Ti is like a grey cave trying to suck in all the light. If the world's welcoming, of course you'll reciprocate. If not, you'll go full edgeshit, collapsing into your own personal spiral of immediate pleasures that cripple you in the long run.
One could just as easily say INTJs are the happiest because they're allegedly the most mature. I'd kill to be that.