Well he didn’t sign The Official Secrets Act like in England. Are agents required to sign Non Disclosure Agreements or something to that effect?
This Former Agent is writing about what when on almost 20 years ago. I think that is a long enough time period for him to be able to say whatever he wants.
Secret Service Agents also are not legally bound to stay silent once they leave government service.
There has always been an unspoken policy to not disclose information about U.S. presidents and the policy is generally upheld with some past minor breeches or two. Plus the Agency's retired-agents organization appears to uphold this "honor" concept also.
However, Secret Service Agents are required by law to speak out if they are witness to any illegal wrongdoing by those they are assigned to protect.
Leni