In general, the Federation has no death penalty
As BMWurm mentioned before me in his comment, the only known case of the death penalty is the General Order 7 mentioned in the question.
In general, the Admiral was right in her statement. It was one, exceptional situation, and the point was, that comparing to the Klingons and their value of (especially alien) life, the Federation has completely abandoned the death penalty.
EDIT:
It should be noted that the contents justification of General Order 7 was classified as "TOP SECRET For eyes of STARFLEET Command only", which would mean "do not disclose to anyone not in Starfleet Command, especially the Klingons, even if they appear to be defectors".
A statement from Admiral Cornwell that the Federation does not have the death penalty, except for one situation that concerns one planet, without liberty to discuss why, would have created doubts and confusion, and would not present the contrast between the Federation and the Klingons.
