I'm not an expert on the palestinians, but it might be different than saying that everyone is an invented people. what it really comes down to is do the palestinians have a culture that is unique and different from other arab states or ethnic groups (again, i dont know). You could say that the chinese are an invented people, in the sense that at some point they all got together and decided they were going to call their language chinese and the land the occupied china, but such a claim is preceeded by the notion that the chinese were different from their neighbors,and in fact they are. the chinese are not the same as the koreans, the british are distinctly seperate from the french, and the americans are not canadians.
The same claim can be made about the Jews, that they never had a country (bible excluded) and therefore are an invented people, but the jews have a distinct and unique history, and are so different from other european ethnic groups that they have been actively persecuted. If the palestinians are more or less interchangeable with other arab nations, then no,they are not an invented people. if they are refugees that no other country will allow despite being culturally 99% identical with other various arab groups, then yes, they are invented