“You can walk straight onto a private jet — there's no security at all.”

On a typical domestic charter in the United States or Europe you arrive at the FBO ten or fifteen minutes before departure, someone checks your ID against the passenger list, and you walk across the apron to the aircraft. There is no metal detector and nobody confiscates your water bottle.

That doesn't mean nothing is happening. In the US, aircraft over 12,500 lb flying for hire fall under the TSA's Twelve-Five Standard Security Program, and larger jets come under the Private Charter Standard Security Program, which requires passenger and bag screening comparable to an airline. Operators also run watch-list checks on international passengers through the eAPIS system, and border authorities can (and do) meet aircraft on arrival.

In the UK and EU, general-aviation terminals apply their own regimes: the crew is responsible for knowing who is on board, and some airports (Luton, Farnborough, Le Bourget) have dedicated screening for certain flights.

So the fair summary: private flying removes the friction of security, not its existence. The person with real authority to refuse you boarding is the captain, and that authority is absolute.