The only way NC headphones block conversations is because they are also acoustically isolating. That is, they would block conversations even with the NC feature turned off. The headphones you link to look like this: they're basically earplugs with headphones built in. Earplugs block conversations because they physically block sound waves from entering your ears. The same principle works with the big over-the-ear cans. It's not the NC feature that's helping, it's the fact that the headphones themselves are isolating your ears from the outside world.
Hmm, interesting. I wonder if it's because they fit in the ear, rather than sitting outside like more traditional headphones. This would allow them to cancel noise at higher frequencies since the driver-to-eardrum distance is smaller.
Have you used other, more traditional can-type NC headphones? Can you compare?