He's using a technical definition of irreducibility [https://en.wikipedia.org/wiki/Irreducibility https://en.wikipedia.org/wiki/Irreducibility] for which the cl

He's using a technical definition of irreducibility [https://en.wikipedia.org/wiki/Irreducibility https://en.wikipedia.org/wiki/Irreducibility] for which the claim is true. It doesn't mean "cannot be expressed more concisely at all".