I don't think it as simple as that. Some of XR do think it's just about carbon emissions and that some nicer green capitalism is the answer (which it arguably could be, but that's another discussion....) but also I do think plenty of people involved in XR do see capitalism as the root problem, or at least they will come to through being involved and developing ideas etc.
Even if they don't, then they're still worth critically supporting etc. as with movements the interesting stuff is often not where they are now, but where they might be going and what they could achieve if their action escalates and gains widespread support, etc. That's one of my concerns with them now, that their use of blockades and disruptions is going to lose them support in the medium to long term.
And also if they were clearly 'against capitalism' how would/should that change what they do?
Oh, and I do think some of the arguments for not being supportive of them as they're not reformist and not against capitalism is slightly double standards from some who manage to support other struggles that are more 'traditional' leftie stuff that also just asks for small reforms like higher wages, better conditions, etc.
BLM is another example, round me loads of their public demands are horrendous tbh, and it just seems a vehicle for promotion of 'black businesses' and personalities scrabbling for power, but to dismiss it simply all as that would be a mistake imo.