As the impeachment saga involving Richard Nixon showed 45 years ago, executive privilege claims are not absolute. With the Supreme Court ruling that the veil of privilege could be pierced in court proceedings, and with Congress threatening to imprison executive branch officials who continued to defy House subpoenas, even Nixon relented and waived his claims of privilege.
In this case, what Bolton knows is directly and fundamentally essential to understanding whether or not Trump abused executive power, perhaps even in criminal ways. Even he must admit, despite his preference for a strong executive, that not all presidential behavior can justly be shielded from the American public. Also, in this case Bolton would not be shielding deliberative processes important for broader U.S. foreign policy — which is what privilege claims are for — but instead would be shielding key evidence of whether or not the president committed abusive acts. As with the Nixon imbroglio, presidential privilege claims are far weaker in circumstances such as these.