If he is just giving a generic talk about lessons learned without explaining the intricate war stories, thats a dull talk. Those are commodities.
In 99.99% the cases where a manager would be so impertinent as to decide upon how the personal time of an employee be used, it's because the manager is a shitty manager and the employee is completely worthless in the eyes of the manager.
It would be a great insult if a manager treated me like that, because it would either imply that I'm too stupid not to know what I can and what I cannot reveal about my work or (the most likely reason) that even though the risk of my presentation hurting the company is minute, I'm so worthless that he thinks he can just tell me what I can and can't do in my free time.
And I'd argue, the employee is tremendously valuable if that is the case, and probably should ask for a huge raise :)