It’s an issue of how to fight it. Those other Gedolim, such as Rav Ahron Kotler for example, opposed Zionism just as much as they did before the war. They just held that after ’48, the battle to prevent Israel from coming into existence is lost, the bad guys won, and we now need to fight Zionism on a different front. The way to do that is to try to obtain Hatzolah Purtah. If we cannot prevent Israel from coming into existence, let’s try to make sure it does minimal damage to Judaism. Frum Jews need to join the government and use whatever leverage the frum community has at its disposal to prevent bad decrees and to ensure the rights and the flourishing of the frum community. They held that joining the State was the best was to oppose Zionism and its bad effects. What changed was, before the State we had to oppose having anything to do with the Zionists. After the State, their goals are already accomplished, so now the best way to fight them is to join their State and make sure they don’t do more damage than necessary.
The Satmar Rebbe held that such an approach is not Hatzolah Purtah but rather makes you an accessory to the crime after the fact. He held that we still need to have nothing to do with the Zionists including joining their state.