How is Dr. Mortimer honest?

Mortimer doesn't disclose all he knows at the Coroner's inquest because he didn't want to publicly succumb to superstition. When briefing Holmes, however, he doesn't hold back and is completely honest.

“In doing so,” said Dr. Mortimer, who had begun to show signs of some strong emotion, “I am telling that which I have not confided to anyone. My motive for withholding it from the coroner’s inquiry is that a man of science shrinks from placing himself in the public position of seeming to indorse a popular superstition. I had the further motive that Baskerville Hall, as the paper says, would certainly remain untenanted if anything were done to increase its already rather grim reputation. For both these reasons I thought that I was justified in telling rather less than I knew, since no practical good could result from it, but with you there is no reason why I should not be perfectly frank.