The opening crawl of ROS makes it clear a transmission has already been made.
The dead speak! The galaxy has heard a mysterious broadcast, a threat of revenge in the sinister voice of the late Emperor Palpatine.
Yet Poe Dameron and all the Rebellion leadership seems caught by surprise when the traitor's information tells them that. From the transcript
REY: What's the message?
POE: We’ve decoded the intel from the First Order spy.... and it confirms the worst. (sighs) Somehow, Palpatine returned.
[All gasp, murmuring]
ROSE: Wait. Do we believe this?
AFTAB: It cannot be. The Emperor's dead.
Why didn't anyone there seem to know about Palpatine's transmission?