I have to believe this is just posturing for the people at home on the part of Zelenskyy. He has to know immediate membership of Ukraine into NATO means direct war with Russia leading to nuclear escalation, as Ukraine would immediately invoke Article 5.

Just saying "Ukraine will be invited to NATO once the conflict is over" is enough; it means whatever territory Russia gets to keep over the course of this conflict is it, because then Ukraine becomes NATO territory. It forces Russia to try and win it all (which they can't) because there won't be another invasion of Ukraine.