In short, that is because "island" is not the only meaning of //.
Besides "island", it can mean "settlement", "arable land by a river", "isolated area", "territory", "turf", "sandbank", etc. Even each section of a supermarket or any sizable store is called . So, **it does not have to be sea water that surrounds a //**.
[]{} was largely lakes and marshes at least as late as the 16th century according to chimei-allguide. So it was a kind of an island surrounded by fresh water.
[]{} is said to have started from a delta according to the same source as above. So, like Fukushima, it was like an island in a river if not the ocean.