The fact that they didn’t rush to do three interviews like this, and then a round of the evening talk shows, was the nail in the coffin. Mental competency is easy to publicly demonstrate, unless you can’t
I don't think it would have made a difference even if he did. The questions of mental competency aren't based on anything real: people are making a lot of hay about an old man with a known speech impediment is slow. If it was anyone else, saying they lack mental capacity would be mean.