Proof.
We will use that \text{Mod}_{(A, \text{d})} is an abelian category with arbitrary direct sums and products, and that these give rise to direct sums and products in K(\text{Mod}_{(A, \text{d})}). See Lemmas 22.4.2 and 22.5.4.
Let M_ j be a family of differential graded A-modules. Consider the graded direct sum M = \bigoplus M_ j which is a differential graded A-module with the obvious. For a differential graded A-module N choose a quasi-isomorphism N \to I where I is a differential graded A-module with property (I). See Lemma 22.21.4. Using Lemma 22.22.3 we have
\begin{align*} \mathop{\mathrm{Hom}}\nolimits _{D(A, \text{d})}(M, N) & = \mathop{\mathrm{Hom}}\nolimits _{K(A, \text{d})}(M, I) \\ & = \prod \mathop{\mathrm{Hom}}\nolimits _{K(A, \text{d})}(M_ j, I) \\ & = \prod \mathop{\mathrm{Hom}}\nolimits _{D(A, \text{d})}(M_ j, N) \end{align*}
whence the existence of direct sums in D(A, \text{d}) as given in part (2) of the lemma.
Let M_ j be a family of differential graded A-modules. Consider the product M = \prod M_ j of differential graded A-modules. For a differential graded A-module N choose a quasi-isomorphism P \to N where P is a differential graded A-module with property (P). See Lemma 22.20.4. Using Lemma 22.22.3 we have
\begin{align*} \mathop{\mathrm{Hom}}\nolimits _{D(A, \text{d})}(N, M) & = \mathop{\mathrm{Hom}}\nolimits _{K(A, \text{d})}(P, M) \\ & = \prod \mathop{\mathrm{Hom}}\nolimits _{K(A, \text{d})}(P, M_ j) \\ & = \prod \mathop{\mathrm{Hom}}\nolimits _{D(A, \text{d})}(N, M_ j) \end{align*}
whence the existence of direct sums in D(A, \text{d}) as given in part (3) of the lemma.
\square
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