Marco didn't tell Zoro this war is for the New Generation.
He only said to Queen and King that they are understimating this invasion, thinking they can't defeat Kaido, because they are PITTING AGAINST THE NEW GENERATION. Marco never says "This fight is ONLY for the New Generation", if he did, he wouldn't have interfered in anyway so far and would have left already, he wouldn't have clashed and tried to control Big Mom, he wouldn't have interfered with Queen's bullets effects, he wouldn't have put Zoro up there, he is a part of this war. This argument doesn't mean anything at all.
Go read that chapter and panel again. He isn't even talking to Zoro, but King and Queen, the ones he is facing,