The purpose of an all-hands meeting is to disseminate clear information to employees. In this case, Yahoo has done a very mediocre job in making the message clear. They could have told their own employees the clear truth about the future of delicious, but instead they couched it in FUD terms like "sunset".
So one employee in particular didn't arrive at the same conclusion about this leaked slide as the Yahoo PR department intended to convey, and leaked the slide to someone outside of Yahoo. That person also came away with a similar interpretation.
This does still point to one of the key mistakes Yahoo makes - it's inability to communicate clearly with it's own employees. And by masking these communication in PR-speak rather than plain clear English either suggest complete incompetance in communication, or a still unclear roadmap for Yahoo in general.
It was fair to assume "sunset" meant close down. Why would an employee of Yahoo leak a slide if it was clear that delicious would merely be sold, not closed down?
"sunset" in this context is a particularly poor choice of word. It has too many connotations of things going down. They could have said instead they intent to sell delicious - their employees do deserve clear and decisive leadership.
This is very much a leadership failure at Yahoo. They have failed to communicate effectively with the Sunnyvale staff who are still reeling after seeing their colleagues laid off.
Unfortunately, speaking as an ex-Yahoo, this is not new, nor surprising.