Imho odds are your manager is right. Are you working for a 3 letter agency, its contractor or subcontractor; are you revealing trade secrets or company priorities, or implying them by revealing your knowledge on a subject; are you using your role as employee of company X to land paid speaking roles; have you signed employment agreements that state any intellectual property you create on or off the job is property of the company; are you speaking at event which could bring suspicion or disrepute to your employer eg blackhat, scammy, or events sponsored by foreign powers; do you work in finance and are being paid an amount in cash or in perks (transport, meals) which invites scrutiny for ethical violations; do you work in a sensitive or "access" position which has required you to sign a code of conduct? What's the reason for denial? Context is necessary.