What "wrong term"? Global warming? Because The Guardian prefers to call it "global heating"? Or am I missing something, because that complaint would be amazingly petty.
Anyway it's not about bringing visibility to global warming to make people aware that it's going on. It's about making a statement. That statement, as I understand it, is "Climate change! Wake the fuck up and do something about it, people!" I don't know if anything will sufficiently get that message through, but it's understandable that they want to try, and painting Stonehenge orange (reportedly in a non-toxic water-soluble paint that will wash away in the rain) seems like a somewhat effective way to get the attention of the news media.