A listener may know a proposition yet behave in a manner inconsistent with it. The speaker can therefore address that person as though resistant to the claim. This rhetorical treatment does not imply the loss of literal knowledge.
Treating an informed hearer as a denier can justify the stronger challenge of negation–exception. Innamā may instead draw attention to an accepted yet neglected truth.
Separate the hearer's knowledge, belief, conduct, and the speaker's intention to explain the choice of form.