Relying on an older lady for security is a poor plan to start with.
Protecting Joe is not in her job description. But she is clearly protective of her husband.
Look what happens with the first protester. She rushes the stage and Jill interposes herself between the protester and Joe. She does not engage, because Security is there to take care of business.
But, a few seconds later, with Security out of position, when a second protester rushes the stage, Jill once again steps between the protester and Joe, and then engages to fend the protester off.
You can't teach that. Some people shy away, and some people fight. Jill Biden, apparently, is a fighter.