If we replace the seminar talk with an MO answer in the above hypothetical situation, I don't think that very much changes.
Maybe you were imagining a slightly different situation, where the sketch in my seminar talk or MO answer fell well short of including all the key ideas. There is a continuum of possibilities interpolating between offering no proof at all and providing an extremely detailed sketch, and I agree that somewhere within that continuum is a grey area where it's not clear that I should get credit for Theorem A. But I think it is common, in situations like this, for it to be possible to give all important details of a proof in a few sentences. In rare cases, the mere statement of Theorem A would be sufficient for experts to easily figure out the proof.
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