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A sheet of graphene is one atomic layer thick. It's very easy to tear apart or pierce with any macroscopic object.

The article is talking about a 'sheet', in the colloquial sense of the word, of the thickness of a piece of cling wrap.

A "sheet" doesn't neccesarily refer to the molecular structure. In this contex, it would mean several layers composited together.

Several layers of graphene put together is graphite, and it's pretty non-magical.