In short, we aren't on track to an apocalyptic extinction, and the new head is concerned that rhetoric that we are is making people apathetic and paralyzes them from making beneficial actions.
He makes it clear too that this doesn't mean things are perfectly fine. The world is becoming and will be more dangerous with respect to climate. We're going to still have serious problems to deal with. The problems just aren't insurmountable and extinction level.
Did you even read the article, Mr/Ms climate scientist?
He’s asking people not to talk like the world is going to catastrophically end once we hit that 1.5 degrees milestone, because it’s making people feel hopeless and apathetic, which is actually slowing our efforts to change.
And he’s totally right. If the government told people a meteor the size of Texas was going to impact earth in 12 hours, there would be effectively zero effort to stop it. If you tune in to a lot of the conversation around climate change from people who are not climate scientists, but who want to leave a better world for their kids and believe climate scientists, they feel hopeless. It feels like a foregone conclusion that we are going to go over the 1.5 degree goal (probably because it is), and if we think the biosphere is going to collapse when it does, it is really, really hard to take action.
It’s not saying to undersell the risks, he’s saying to be truthful about the risks. We can definitely still salvage complex life on earth with optimistic, consistent effort, but recent media coverage has been giving the impression that it’s too late. This is bad and counterproductive.