You've jumped all the way to trying to micromanage your boss, which is inherently a politically explosive undertaking, but where you actually should be is just asking much more straightforward questions: why don't they want you to give the talk? Maybe they're trying to sabotage your career progress, but maybe there are legitimate concerns about the content of the talk (e.g., exposing secret sauce internal methods to…
My advice to OP: ask to walk through your presentation deck with your manager, both to get feedback and buy-in. Managers will be more invested in your success if they feel invested in what you're doing. (Suggestion: offer to plug your company's job openings with your manager's contact info.)