[waluigi]

I think what the poster above is saying is that by saying "the pentagon" the author is referring to the (unique) 5 sided regular polygon, as opposed to some random, potentially non-regular 5 sided polygon.

[thaumasiotes]

That would not stop it from being a bizarre error in an otherwise decent article. There is no such use.

I interpreted it in the same way as e.g. "the cow has four stomachs".

This is a question about graph theory anyway; there are no side lengths or interior angles.