What is meant by “It’s Penelope” in the poem Television?

It is not confirmed what the poet actually refers to by that. I think ‘Penelope’ is a character of a popular book, may be the faithful wife of Odysseus in Homer’s Odyssey. 
 
The poet has said this while talking about the love of book in the children of the past when there was no television. The quoted line above is an example expression of a book-lover of his time when they found both the smell and the content of the book attractive and thanked God for presenting such a book to them.

Penelope is a girl’s name; she was a mythical character who was eaten up by the cannibals, people who eat human flesh.

Penelope, in Greek mythology, a daughter of Icarius of Sparta and the nymph Periboea and wife of the hero Odysseus.  Homer’s Odyssey tells the story of how, during her husband’s long absence after the Trojan War, many chieftains of Ithaca and nearby islands become her suitors. The cannibals here are the chieftains who are eyeing Penelope with hungry eyes.

Our teacher explained it to us that she was a mythical character who was eaten up by cannibals.

In the above line the poet is adding an extra information.

The line before this says that cannibals were crouching round the pot. Therefore when the pot is smelled from afar, it smells good. But then to their surprise, they find Penelope!

This is the reason why these lines were given in brackets in the original version.

Thank you! Hope you got your answer!
P.S. This is the meaning of the line, not Penelope.