I've seen a very good QA engineer leave a company in exactly the same circumstances.
She wanted to do an external presentation on cutting edge security testing (nothing proprietary, just cutting edge) and the idiot manager wouldn't let her.
She asked why and was given bullshit reasons and so she asked her manager's manager which esentially let her do the presentation and agreed with her she was given bullshit reasons.
Nevertheless her direct manager kept a grudge and she eventually had to leave the company, I'm sure she went to a much better place after that.