The event that took place in Jaffa in 1921 was definitely a riot and not an act of genocide. Here are some excerpts from the Wikipedia source you quoted:
About 45,000 people lived in Jaffa at the time, roughly half of them Muslims, a third Jews, and the rest Christians.
The first shots had apparently been fired to disperse a procession marching from Jaffa to Tel Aviv without a permit. The parade had been organized by the Jewish Communist Party, officially called the Socialist Workers Party, though its opponents used an acronym of the party’s Hebrew name to nickname it “Mops,” which means “pug dog” in German. The previous night the party had sent boys out to distribute leaflets in Arabic and Yiddish emblazoned with slogans calling on the workers to topple the British regime and establish the Soviet Union of Palestine.
Meanwhile in Tel Aviv, a large May Day parade had been organized by Achdut HaAvoda, the major Jewish labor party at the time, and sanctioned by the authorities. Tensions ran high between the rival parties. At some point the communists and Achdut HaAvoda people ran into one another, and a fistfight ensued. The police chased the “Mopsies” members back in the direction of Jaffa, where the Communist parade clashed with Arabs, who were equally unsympathetic to a Soviet Union of Palestine.
Dozens of witnesses—Jewish, Arab, and British—all told the same story: Arab men broke into Jewish buildings and murdered the occupants; women came afterward and looted. Bearing clubs, knives, swords, and in some cases pistols, Arabs attacked Jewish pedestrians and destroyed Jewish homes and stores. They beat and killed Jews, children included, in their homes; in some cases they split the victims’ skulls open.
Arab accounts of the Jewish violence are very similar to the Jewish testimonies about the Arab riots. The Jews looted homes and stores. They broke into Arab houses, beating and killing the occupants; in one house, a woman and child were murdered. A hunchbacked Arab and his children were killed in an orange grove; their bodies were disfigured. A Jewish policeman took part.
A commission of inquiry later appointed to investigate the riots found that the fight between the communists and Achdut HaAvoda was the spark that lit the fire and described the riots as “an orgy of pillage.” Many witnesses identified their neighbors among the attackers and murderers; in some places Arabs had come to the defense of Jews and gave them refuge in their houses.
A total of 47 Jews and 48 Arabs were killed in the disturbances, and the wounded numbered 146 Jews and 73 Arabs.
There is no evidence that the Jaffa riots were premeditated. Arab leaders and spokesmen, first and foremost Musa Kazim al-Husseini, condemned them.
The Arabs put together a petition that they submitted to the League of Nations, in which they expressed their grievances; the essence of their demands—independence and democracy—remained unchanged through the end of the Mandate.