I think you might be narrowing the definition of politics. Something is political not just when two arbitrary political “sides” disagree. Something is a political issue when government policy is involved, and Greta is absolutely in the business of changing government policy. Climate change is also a political issue because it does create a divide among political groups: the rich and the poor. The people who own the most stuff will profit from irresponsible pollution, and have the most means to avoid its consequences. They will be using their political power to make sure things stay that way. The poor will suffer.
I think you might be narrowing the definition of politics. Something is political not just when two arbitrary political “sides” disagree. Something is a political issue when government policy is involved, and Greta is absolutely in the business of changing government policy. Climate change is also a political issue because it does create a divide among political groups: the rich and the poor. The people who own the most stuff will profit from irresponsible pollution, and have the most means to avoid its consequences. They will be using their political power to make sure things stay that way. The poor will suffer.