I agree and disagree with this. I think that it is hard for adults to learn a new language, but not in the way that you think. Children can learn a new language easily not because they want to but because they have to if they want to be able to communicate with the people around them and let people know what they want.
Children have more leisure time to learn a new language as well, and the first thing that they learn is trying to speak the language and not trying to create sentences, depending on their age. In time, they have to read and write in that language. Adults have other responsibilities other than trying to learn a new language, so they don't have time to spend copious amounts of hours trying to speak a new language. However, adults already know how to read, write, create sentences, and have knowledge of what objects to connect words with.
Children are learning more than just a language; they are trying to learn about the world and how it works all at once. Adults already have this advantage, so it's hard to think that a child would come out of learning a language infinitely better than an adult.
If an adult who doesn't speak English wants to learn and they live in an area where English is the only language spoken, they would have to learn, and they would learn more vocabulary than a child.
An adult who doesn't speak English and stays within their community of people who speaks their language wouldn't have to learn English, but if they didn't have anyone that speaks their native language around them, they would be forced to learn English.
There are so many factors to consider, so I agree and disagree.