You seem to be suggesting that there's a "right" way to participate in democracy. Instead of passing down armchair judgements about which forms are the wrong way to participate, could you perhaps suggest what you think would be a better alternative to what XR has planned here?
> Why would you buy something that itself has a carbon footprint ...
I'm curious as to what point you're actually trying to make here. Why do people chain themselves to bulldozers to protest mass deforestation? Why do people perform acts of civil disobedience to protest nations fighting wars? Most of the time the people who are involved in those movements are fundamentally morally compromised. For example if you're protesting the Vietnam war and the general US war mongering as an American, you're doing this from a place of extreme privilege - much of which was afforded to you precisely because of the great success the US has in previous wars.
> What do you want people to do, fly only battery-powered passenger airlines?
This is a bad faith interpretation of what the activists are trying to do. They're not protesting commercial air travel (although I'm sure many of them have strong opinions against our frivolous use of it). They're trying to wake people up to the fact that, while somewhat contentious, there's a pretty large (and growing!) consensus that we're utterly trashing our planet and hurtling headlong into catastrophe.