English is not my native language (French is), but as virtually anyone in engineering has to, i learnt it. It didn't take that long or much effort, no real format training...just guessing at the word's meanings when playing video games and watching TV and eventually I picked it up... now I live in the US and most people can't tell I'm not a native speaker unless they see my first name. My writing could be better, as you probably can see, but I'm even worse in French, so its more that I suck at writing in general.
I tried learning Chinese (my wife is Chinese, so I'm immersed in it a lot). Unless you know a lot already, guessing the meaning of a written word is almost impossible, since its pretty binary: either you know that character, either you don't. People who know a lot can somewhat guess from comparing against all the words they know, there's some patterns, but still. It obviously also affects writing, though you can use input systems that use latin characters...you should won't know if you made a mistake.
Talking though, you don't have that issue, and the grammar is easy, but the tone system is very foreign to someone only used to western languages. Thus what happened as per the summary. He was able to speak, but sounded like a kid. Learning how to speak it is easy, but not sounding like a westerner is very difficult. At least it wasn't Cantonese
Thats probably what they meant.