If one does an internet search for the words Extinction Rebellion and Communist, or Socialist, you do get hits, more than a couple of newspaper articles.
The Communism practised in the Soviet Union and many other places was historically not good for technological innovation, or fostering creativity, nor for raising the living standards of its citizens. Whatever you want to call what is currently being practised by China still isn't as good at creativity as the Western system, but it is catching up and in some areas surpassing the efficiency of Western Capitalist Democracy.
There are quite a few people in the US who think of the UK as Socialist, because we have the NHS and other social safety nets. There is clearly a spectrum of what people consider to be an acceptable level of socialism, and how effective these levels are.
Now in the case of environmental action, we are in the position we are in large part because people, and companies, have been free and encouraged to do what they want, freely, and to pursue profits competitively, because the model said this was the best way to foster creativity, drive innovation and raise living standards. Can people and companies choose to change direction, when such a change may not be comfortable, and is surely not going to be profitable, at least to begin with? If you are the first company, and you change to a more costly but more environmentally friendly system, while your competitors do not, you risk ruin.
In comes the "communist" idea. The people are pointing out that perhaps the only way to get the radical change they see as necessary, is for governments everywhere to act and force everyone, globally, on to a new path, force change for the common good. Of course, the worry for governments is much the same as for countries. If they bring their country to a halt with energy rationing, or similar radical changes, and competing countries keep burning coal a bit longer, they risk bringing ruin on their industries and people.