Well other than the thirteen or so he himself admits to?
That statement was made in an interview the Abu Daoud gave to Sports Illustrated, back on August 26, 2002--after having mentioned it in his memoir, Palestine: From Jerusalem to Munich, in 1999.
Just as there is no proof that Abbas knew that he was providing the financing that made the murder of Israeli athletes possible, there is no direct proof that he had any part in the murder of 2 US diplomats back in 1973.
In his 2007 article, How Arafat Got Away with Murder: The State Department covered up his responsibility for the 1973 slaughter of two American diplomats, Scott Johnson writes about Arafat's direct connection to the murder in Khartoum of U.S. ambassador to Sudan Cleo Noel Jr and U.S. Deputy Chief of Mission George Curtis Moore on March 2, 1973.
Johnson describes how on March 13 1973, Secretary of State William Rogers wrote in a cable:
Question of link between Black September Organization (BSO) and Fatah has been subject of much public discussion since murder of U.S. diplomats in Khartoum. Fatah leader Arafat has disavowed connection with BSO...The Black September Organization (BSO) is a cover term for Fatah's terrorist operations executed by Fatah's intelligence organization. . . . Fatah funds, facilities, and personnel are used in these operations. . . . For all intents and purposes no significant distinction now can be made between the BSO and Fatah. . . . Fatah leader Yasser Arafat has now been described in recent intelligence as having given approval to the Khartoum operation prior to its inception.For a variety of reasons this conclusion has been ignored and denied for decades, but even today--so many years after those murders, there are implications, as Johnson concludes:
The publication of the 1973 CIA summary ends 33 years of public silence on Yasser Arafat's murder of two high-ranking State Department officers. It is a notable event. The tortured history of the government's treatment of Arafat's responsibility warrants much additional investigation. And given the fact that Arafat's right hand man is the current prime minister of the Palestinian Authority, it is not only of historical interest. Speaking in a 2003 interview from the perspective of an average citizen who was also a firsthand witness to a most significant piece of this tortured history, former NSA analyst Welsh may appropriately be given the last word, at least for the moment: "There are limits to which foreign policy issues should require a man to lower himself. Shaking the hand of a murderer of a U.S. ambassador is such a case. Any peace based upon that hand is a delusion."As Palestine Facts notes, Abbas was very close to Arafat for decades:
Mahmoud Abbas was a founding member of the Palestinian National Liberation Movement (Fatah) present, along with Yasser Arafat, in the 1964 beginnings of the terrorist organization dedicated to the destruction of Israel. Since 1968 he has been a member of the Palestinian National Council (PNC) and rose to Secretary General of the PLO Executive Committee. The Executive Committee and the PNC are the top two decision-making bodies of the PLO... Abbas has been Arafat's top deputy for nearly 40 years.It won't be asked, but it is a fair question to ask what Abbas knew about the planning of the terrorist operation in Khartoum--and of the murder of the American diplomats.