In the middle of the article, he writes:
Biden is the second Catholic to become President of the United States after John F. Kennedy, who did not tolerate Ben-Gurion at all. [Kennedy] had threatened him in 1963 to impose a boycott on Israel if it did not commit to disclosing its nuclear program, and his assassination was carried out by unknown hands a few weeks later!
Kennedy did want to make sure that Israel's nuclear program would remain peaceful and there was a series of letters between US and Israeli leaders from 1962 through after Kennedy's assassination. I am unaware of any threat to boycott Israel by the US, and that seems to be made up.
Husseini's clear implication, though, is that Israel was behind the murder of Kennedy.
Which fits in well with the conspiracy mindset of many Palestinians.
(h/t Ibn Boutros)