To call someone a liar is very pejorative and unprofessional.
They didn't call him a liar in that segment, they simply showed viewers that he lied, and told him what he had actually said—which he continued to lie about. Do you know what is really unprofessional? An MP lying on camera. That's Johnson. You're supporting a liar.
"One day, we shall die. All the other days, we shall live."