I imagine their maths is fine. Their objective is to tackle climate change through raising awareness. That happens at the macro scale.
They didn't think through the ecological results of someone scraping off the destroyed concrete and pouring more.
It's far grimmer to realise that people know, they just don't think we have an acceptable answer yet.
I'm not willing to accept any climate action, if it means I can't go on holiday abroad or it makes my life meaningfully more expensive etc.
It's selfish, but we need to solve it in a way that makes people globally richer.
(This is one of the great things about renewables coming down in price)