Japanese Reacts to All US State Accents !!
= new reply since forum marked as read
... couldn't distinguish various accents within the USA.
I suspect it means they never developed a good grasp of the language, struggling to understand many words in the first place.
Years ago, a Vietnamese-American coworker asked me what "redneck" meant. Part of my explanation involved a rural / Southern accent. Then he said that he couldn't tell any difference. So then I exaggerated the accent even more, and he again said it sounded the same to him. I later found an online video of Southerners talking with very strong accents, but it was "all the same" to him! Then I found some strong Boston accents online too, and he again said it all sounded "American" to him!
That guy misunderstood words so often, accents were apparently washed away in the waves of confusion for him.
I generally used a kind of "baby talk" around him, to help him understand me. One day, I was still talking that way after work to a convenience store clerk! I then spoke normally after seeing the clerk's perplexed expression! 
The Japanese man in England, hosting the video, clearly has a far stronger grasp of the language.