The point people bring up about concrete CO2 use is absurd. To me it screams of refusing to talk about the debate the protestors are having and instead trying to find any minor nit to pick, which then lets you declare that they're imperfect and therefore not even worth discussing.

In the grand scheme of things, the concrete use here is completely unnoticeable. They're not causing extreme ecological damage here. This is a protest that does something about the problem they're complaining about, slows down their enemies and brings a lot of publicity to their act of protest. Without making a value judgement, this is much better than what a lot of other protests achieve in terms of direct effectiveness.

Frankly, the CO2 retort can be reused for almost any protest that destroys or defaces something, because the work to undo or replace that probably creates emissions.