I was thinking, every cataclysm subtracts a dimension from humans? like, before the last everyone was 3d, then now 2d. so with the next 2 humanity will become 1d/0d?
My interpretation of this is that the previous Great Cataclysm is the one that destroyed our current world (IRL) and the world of Fire Force is basically the future of Earth after the Cataclysm. That's why they used real life photos to show it. Also, her whole monologue was explaining that mindless use of technology was the cause of that Great Cataclysm 250 years ago.