Energy calculation for a short circuit of a battery
From the datasheet, the operating discharge current is given by the opertaing discharge voltage and internal resistance which is: $$I_d=V_d/R_i$$
From the datasheet your discharge voltage is 2.8V @25°C and the internal resistance is 0,45 mOhm which gives you a discharge current of 6223 A.
But, the maximum discharge voltage is when the battery is charged at 100% if your battery is fully charged at 3.5V, then your calculations are good.   Note that a 3CA means that the discharge current will discharge the entire battery at the rate of 300A in 1 hour.