I would not encourage the attempt, nor suggest that someone should. It was just an observation from the Portland riots during the summer. Antifa was being organized and trained.
The concrete itself would not burn. However extreme exposure to intense heat would structurally weaken it. And there is probably a lot of flammable material inside. It would need an "accelerant", and iirc, Antifa would disable the water into the building to effectively disable the fire sprinklers.
https://www.quora.com/How-can-fires-burn-concrete-buildings?share=1
The sarcasm is strong with this one.