Is this an error in Oppenheim and Schafer's Discrete-Time Signal Processing?
It is not a mistake. In section 4.8.4 the authors introduce the reconstruction of $x_r(t)$ from $x[n]$ in equation $(4.140)$; and in the paragraph below, the authors consider the input sequence and output signal to the ideal D/C converter as $x[n]$ and $x_r(t)$ respectively, as shown below
$$x[n]{\longrightarrow}\boxed{\textit{Ideal D/C converter}}{\longrightarrow}x_r(t)$$
Which will be more in line with the last input-output block of Figure 4.47(a) shown as
$$\cdots \quad y[n]{\longrightarrow}\boxed{\textit{D/C}}{\longrightarrow}y_r(t)$$
To clarify, the extension from ideal D/C converter to the D/A and their corresponding maths with inputs-outputs relations and the figures, please consider reading the full section 4.8.4 with emphasis on the change of notation just after equation $(4.140)$.