Some people think that keynote speakers techniques are some kind of collective hypnosis. Is that true or this is a myth?

This is a myth.  Keynote speakers, unless trained as hypnotists, are not capable of capturing an audience in that manner.  Keynote speakers do, in many cases, want to “wow”  their audiences, but the practice of hypnosis is not used.  The goal of the keynote speaker is to use their speaking capabilities and storytelling abilities to captivate their audience with meaningful and relevant content.