Given that Ukraine and NATO knew by April Russia intended to take the entire country, the Russian army was outside of Kyiv, why should Zelensky have trusted Putin to honor any agreement?
Any deal Russia offered was just to by time for their army to regroup and try again. When it comes to Russian irredentism and the 2nd CCCP, Putin can not be trusted.
Ultimately, that is probably why Zelensky declined. Russia will learn from their mistakes and launch from their bases in Crimea and Donbass again.