"I can only conclude that people pushing this narrative either has a problem with decolonisation and land back as a concept, or with Jews, specifically."
It the concept of "land back" itself.
The term "land back" is an appropriated term that has been used since the late 17th century for indigenous North american tribes (people dont seem to give a fuck we even exist btw, or if they do its racist). I have been to countless protests to protect or regain tribal land, peoples home that was taken by European colonial efforts. We go through extreme racism, prejudice, violence. We are abused in all government systems. No one speaks out for us first nations. They tell us "you have your rezes youre fine stfu parasite".
I have to hear people say "fuck Canada and USA bring an intifada!" Which literally means they want to give land they stole, not back to us the original people, no they want to give it to another genocidal regime that will finish what Christianity started. They dont even consider that this isnt their land to give, however, here we are.
They dont want land back for anyone, because if decolonization happens then they literally expect native people to have a violent, and bloody "uprising" giving them "pay back", or making them leave (which isnt really a part of the landback movement).
Many native praise Israel and Jews for a successful decolonization effort, and we dream of a similar ideal. However, this would/could adversely affect the people with extreme (and unchecked) privileges and that makes them feel uncomfortable.
Western society has a hiarchy based on race, and those at the top right now are people scared of decolonization.
I get constant harassing and offensive asks just for being native, from being called an animal, to being called a parasite on the government, and it gets even worse. These people are the same people who support tyrannical regimes.
So as a native person, from just that perspective alone. Its the actual threat of decolonization of a majority of western society (basically anywhere outside Europe).
As being also Jewish, id also say antisemitism has always been a problem, people fear those who are not like them, fear brings out some crazy emotional issues.