A 1985 U.S. Supreme Court ruling declared Long Island a peninsula.
So now, legally, it is a peninsula.
But of course, the Supreme Court is not actually supreme.
Their ruling did not change reality.
A fleet of dump trucks filled with soil visiting the East River* could have done it.
But that didn't happen, did it? Nope.
Long Island is still completely surrounded by water, even at low tide.
What the ruling did accomplish was to keep Block Island Sound from coming under federal control.
Weird, huh?
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*Did I mention that the East River is not actually a river? It's an estuary.