In Obi-Wan's Order 66 warning transmission why didn't he tell the surviving Jedi about the Sith, including Palpatine's true identity?

If you've seen the premiere episode of Rebels you will have seen Obi-Wan's warning message that he broadcast to Jedi throughout the galaxy in RotS. In this message he speaks cryptically & warns of a "dark shadow" falling & to avoid the temple & await a "new hope".

Wouldn't it have been kind of useful to inform the Jedi that their mortal enemy the Sith had returned? & specifically that Palpatine himself was a Sith? You'd assume that the majority of the Jedi who were not in the loop of the Council would be unaware of Darth Maul & then also the Council's suspicion of a Sith Master existing somewhere. Yoda mentions a few times that they should keep this under wraps. I think now would've been the time to reveal the truth. The more info the Jedi had the better. Who knows if the likes of Kanan knows anything about the Sith's return! I find Obi-Wan's omission of this information bizarre. What do you think?