The constitution at the very least is a paper document that Zelensky can’t ignore OPENLY. So he’s saying that he would need a referendum if Ukraine wants to give up any territory to Russia.
The same way Putin can’t send conscripts outside Russia’s borders due to Russia’s constitution. Putin can fig-leaf it by calling this a “special operation” and forcing some conscripts to sign papers.
Zelenskyy can’t find a way to fig leaf giving away Ukrainian territory to Russia without a referendum.
It would, also, be national suicide for Ukraine to give up the energy and industrially rich Donbas and their southern ports.