The Stacks project

Proof. As $\mathcal{E}^\bullet$ is a bounded above complex of flat modules we see that $\mathcal{E}^\bullet \otimes _{\mathcal{O}_ X} \mathcal{F} = \mathcal{E}^\bullet \otimes _{\mathcal{O}_ X}^{\mathbf{L}} \mathcal{F}$ for any $\mathcal{O}_ X$-module $\mathcal{F}$. Hence for every $\mathcal{O}_ X$-module $\mathcal{F}$ the sequence

is exact in the middle. Since $\mathcal{E}^{a - 2} \to \mathcal{E}^{a - 1} \to \mathcal{E}^ a \to \mathop{\mathrm{Coker}}(d^{a - 1}) \to 0$ is a flat resolution this implies that $\text{Tor}_1^{\mathcal{O}_ X}(\mathop{\mathrm{Coker}}(d^{a - 1}), \mathcal{F}) = 0$ for all $\mathcal{O}_ X$-modules $\mathcal{F}$. This means that $\mathop{\mathrm{Coker}}(d^{a - 1})$ is flat, see Lemma 20.26.16. $\square$