That the energy output of a flame is regulated by the oxygen flux. Phlogition theory suggests a phlogiston flux that is just the oxygen flux with a sign reversed and consequently when they measured the mass of the phlogiston, they got a negative value.
I don't agree that the essence is not wrong, but I can imagine a decent argument for that position predicated on considering Phlogiston as chemical energy. Some substances can easily release energy through oxidation, others not so much.