Even if this was some type of planned provocation by Zelensky, there's still no reason to give him that moment on live TV.
You could just cordially disagree with his assessment, and acknowledge that some issues will still need to be addressed. And if necessary, chew him out in private, if he did indeed lead Trump and Vance on privately, and then change his tune in front of the media.
An experienced negotiator ought to be familiar with such tactics. Dude has been dealing with interviewers who've pulled that stuff on him for decades. Trump isn't supposed to be some big baby who needs everyone to cater to his needs, or otherwise he "loses his cool". You don't get to "lose your cool" as the most powerful man in the world. You need to be smart, calculating and patient.
It seems to me like Trump got a little too used to his "fast food" approach to resolving conflicts, which was to just threaten countries with tariffs or use executive orders to get his way. This situation is going to require a lot more effort invested into it. He's not going to get his Nobel Peace Prize for free. He's going to have to work for it. Trying for about a week and then throwing your hands up because someone didn't agree to your initial suggestion, that's just not going to cut it.