The most interesting talks in conferences usually are case studies or explain how X was successfully used in a pratical project. It might be hard for OP to give such a talk unless he talked about his personal projects only.
If he is just giving a generic talk about lessons learned without explaining the intricate war stories, thats a dull talk. Those are commodities.
Oh please, unless he developed something patent-worthy or something specific that could be used by direct competition then it's stupid not to let an employee talk about his (technical) work.
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.
You're arguing against a strawman. In most cases, yes the manager probably is using wrong judgement, but there are valid cases where an employee would be disclosing information that could be used by competitors.
And I'd argue, the employee is tremendously valuable if that is the case, and probably should ask for a huge raise :)
If he is just giving a generic talk about lessons learned without explaining the intricate war stories, thats a dull talk. Those are commodities.