You're shifting goalposts and conflating two different groups with different ideas and tactics.
Just Stop Oil activists protest in museums with timeless paintings with great cultural and historic significance. They take care that their actions don't irrevocably harm the art. The priceless quality of the art is essential to the message of the protest, as it contrasts with the priceless nature of what climate change is in the process of actually destroying.
The anti-genocide protester damaged a portrait of a British statesman displayed on the wall of a public area of Trinity College. This is part of a conceptually distinct form of protest where activists challenge public monuments to people with tainted legacies. The artistic merit of these products were pedestrian even for their time, and merely being old does not endow them with intrinsic cultural value. People concerned about the preservation of similar works have moved them to museums where their public display is less likely to be interpreted as an endorsement of their subject's legacy. One could argue that a greater artistic value comes from the creative defacement of these publicly displayed political advertisements that have long-since outlived their historical moment.
Do you carry the same outrage toward the destruction of monuments to Confederate commanders or defacement on Nazi memorials?