Imaginary in what sense? They are backed up with a network of computers all having access to all the transactions.
If anything is imaginary, it's the value of regular paper money. It's basically people in power telling you it's worth X amount and you don't even know how much of it exists while cryptos are generally transparent and decentralized.
i think he mean something like this, pretty simple, i imagines to have x amount of coin and i give you this x amount for your y amount of coin, you then give me that y amount for x amount and we do a trade, we write this on paper of course, and we give them a value, other people do the same and we create an immaginary netowrk for our immaginary altcoin, but this can't work because someone could simply imagines to have an amount that does not belong to him, it's too damn easy to cheat and change the rule