Do you have a link? Hawking’s office this morning said he wasn’t boycotting, he was just in the hospital which is why he didn’t attend.
At this time, it doesn’t seem clear if Hawking was boycotting and retracted, never was boycotting, or still is.
Israelis furious over Stephen Hawking’s conference pullout over Palestinian boycott call
Later in the day, the university corrected itself. “We have now received confirmation from Professor Hawking’s office that a letter was sent on Friday to the Israeli President’s office regarding his decision not to attend the Presidential Conference, based on advice from Palestinian academics that he should respect the boycott,” a Cambridge spokesperson said.
The Washington Post has a similar story saying the retraction was wrong.