Why can it be easy?

Recently, I've seen advertisement of English textbook with the phrase "It's super easy to learn English" on this webpage.
The reason why such an advertisement is shown would be the quantity of my English sentences here.
But, for those who are learning foreign languages, it's impossible to learn without spending much time.
Some people say "Oh, as he/she spent childhood in the United States, he/she can speak English fluently."
This opinion would let speakers disappointed. They can speak, not because they just stayed there, but because they studied harder.
On the other hand, there are lot of people who can't speak local language, though they've lived where the language is spoken.