I think the argument is subtley different (sorry if this is nitpicky)
1. Palestine is a state, whose territorial extent includes the gaza strip (the most controversial proposition)
2. Under international law, a soveriegn state has the right to prosecute any crime that takes place on their territory. In many ways this is kind of the definition of soveriegnty - the ability to control and make decisions in your territory (in the caee of war, subject to the restrictions imposed by the geneva convention)
3. Soverign states can delegate this power to anyone they chose
4. Palestine delegated this power to the ICC, subject to the provisions of the Rome statue.
> So, according to the ICC, you don't need to be apart of the Rome Statute for the ICC to have jurisdiction
The idea that courts have juridsiction over foreign nationals who commit crimes in their territory is very standard and is generally true for all courts.
E.g. if you are a tourist visiting another country and murder someone, you still get arrested by local authorities. There is no get out of jail free card because you are a foreigner. What is relavent is where the crime took place not who comitted it.
In the case of the ICC, the ICC is acting on behalf of Palestine. So its juridsiction would be the same as whatever Palestine's would be minus any additional restrictions imposed by the rome statue.