All armies rely on good communications and a rebel army is the same but the difference most of them don't have advanced communications equipment like the Syrian army does and most fighters rely on their cellphones.
Destroy the cellphone towers and cut landlines in rebel held areas you cut off communication capabilities and overnight your enemy is weakened.
Cellphone towers are easy targets ( they don't fight back & they are not protected ) and the result is very effective.
He reminds me of an Artificial Intelligence in some game that would threatens me and orders me to surrender when the AI's army is already decimated and almost defeated.
You know what happened ? The AI lost the game.
Its not so stupid
Whilst the chances of it having any effect are low - zero what are the cost financially or in military recourses to send a text?
If this text can convince even 1 rebel that the game is up don't get yourself killed then it was worth doing.