It's not a monopoly that's important here, just willingness and ability. Jimmy-the-Stereotype is but one entrant in the free market of violence but he can still force you to liquidate your assets unless you have a grudge against your own kneecaps.
You can hire Johnny-the-Mobster to protect you from Jimmy-the-Stereotype, but not against Uncle Sam or Grandpa Vladimir. State actors still have a disproportionate power.
Sure but Johnny-the-Mobster isn't exactly clever math either.
The term "monopoly on violence" refers specifically to legitimate violence.