The first half of the objection assumes that a necessary truth could never be denied. But immediate knowability and universal, flawless understanding are not the same claim.
Disagreement may stem from misunderstanding the terms, misconstruing the proposition, deliberate rejection or some failure of comprehension. The principle “the whole is greater than its part” illustrates why the relevant concepts must be correctly understood before classifying an alleged disagreement.
A disagreement alone therefore does not prove the nonexistence of necessary knowledge. One must ask whether the parties dispute the proposition itself or understand it differently.