Being stateless (flagless, without national character) means that any state is authorised to exert jurisdiction over a stateless vessel.
Under whose law? How can whatever law rightfully be applied to a party that refuses to acknowledge the purported jurisdiction or authority?
UNCLOS and customary international law.
> How can whatever law rightfully be applied to a party that refuses to acknowledge the purported jurisdiction or authority?
That's in effect what international law says. No international law protects against against states that mess with stateless ship. Positing oneself outside a jurisdiction or authority in international waters removes legal rights and protections.
"We are outside the law" and "We have legal rights" can't be combined together opportunistically.
Humans in a vessel have basic human rights, but "states", "corporations", and "properties" are made up things established by laws and treaties. If you are not party in any of them, they don't exist.