When the same flag was raised after the assassination of general Qasem Soleimani by a US airstrike, AFP did a fact check and reluctantly admitted that it really is a call for revenge:
According to Hossein Abadi, ..it is rare for the red flag to be raised on this mosque outside the month of Muharram. “Due to the martyrdom of general Qassem Soleimani and his comrades, and after hearing the supreme leader calling for a harsh revenge, we unfurled this flag so that all (Shia) believers in the world and all the freedom fighters gather around this flag to avenge Qassem Soleimani’s blood unjustly shed,” Abadi told AFP.The flag bears the Arabic phrase, “Ya la-Tharat al-Hussein,” which can be translated in English as, “O ye avengers of Hussein.”According to certain Iranian Shia traditions, “Ya la-Thara al-Hussein,” should be a rallying cry for believers upon the return of the Hidden Imam, the last of twelve imams recognized in Imamiyyah, the largest branch of Shia Islam.
Sky News Arabia says that raising the flag of revenge is considered in Iranian heritage as a “declaration of a state of war.”
In fact, as I've noted previously, in 1877 there were very similar newspaper articles about a fearful green flag of jihad being unfurled in Istanbul:
