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I would guess their argument/belief is that it's far too late for "sympathy" to make meaningful changes.
There will be plenty of people using the tube who will have ignored all previous mentions of climate change and the issue society face because of it, and direct action is the only way to make them notice.
(N.b. I don't agree with this POV, just know someone fairly involved in it and would imagine that's the response I'd get)
I think disrupting the tube is going to lose them a lot of sympathy. They should have just focused their resources on the existing locations.