I wonder if there is typo in your quoted sentence.
If there is no typo in that sentence, it would mean that:
"Only parts of old Japan were islands. (The main islands were a peninsulaor even a part of the continent itself at that time, so) people were easily able to come from and go to the neighbor countries (on foot)."
It seems a very old story and I wonder if any human beings existed at that time.
Does this explanation make sense in your context?
Well, even if it's an incorrect choice, it's not an incorrect sentence. I assume your multiple choice question addresses your reading comprehension of a piece of Japanese text. If so, wrong choices are wrong in factual terms, and not grammatical terms. It makes no more sense and is no less correct than the famous:
Colourless green ideas sleep furiously.
But showing the background of your enquiry would have saved some confusion.
I thought it is not quite necessary, as I was sure that grammatically it's correct, and it's just a wrong opinion, wrong point of view, and it's very usual, if somebody says a mistaken thing)). For example, it could be a part of answer of a schoolboy, or an opinion of an alternative scientist, or a part of a fantastic novel...
But yes, I agree, that without context it may lead to a confusion, so I'll pay attention to this in future.