Maybe, go one step further and consider what the difference between $C_\mathrm{s}$ and $C_2$ really is?
Here are the character tables for both point groups:
$$\begin{array}{c|cc|cc}
\hline
C_\mathrm{s} (= C_\mathrm{h}) & E & \sigma_\mathrm{h} & & \\ \hline
\mathrm{A'} & 1 & 1 & x,y, R_z & x^2, y^2, z^2, xy \\
\mathrm{A''} & 1 & -1 & z, R_x, R_y & xz, yz \\
\hline
\end{array}$$
$$\begin{array}{c|cc|cc}\hline
C_2 & E & C_2 & & \\ \hline
\mathrm{A} & 1 & 1 & z, R_z & x^2, y^2, z^2, xy \\
\mathrm{B} & 1 & -1 & x, y, R_x, R_y & xz, yz \\ \hline
\end{array}$$
The terms to the right describe rotations and translations in 3D molecules, and the harmonic oscillator isn't a 3D molecule, so these are irrelevant. Once we strip that away, we're left with the conclusion that except for the choice of labels, $C_\mathrm{s}$ and $C_2$ are really equivalent.
In group theory terminology they are isomorphic.
- There are two symmetry operations, the identity operation $(E)$ and another one $(C_2$ or $\sigma_\mathrm{h})$
- There is one irreducible representation which is totally symmetric $(\mathrm{A'}$ or $\mathrm{A})$
- There is one irreducible representation which is asymmetric with respect to the non-identity operation $(\mathrm{A''}$ or $\mathrm{B})$
Sure, the labels are not the same, but we can only draw a clear distinction between the labels in 3D anyway. In the case of the harmonic oscillator there is only one non-trivial symmetry element and that doesn't change whether we call it $\sigma_\mathrm{h}$ or $C_2$. (This echoes the point in Hans Wurst's comment.)
So, it's entirely up to you whether you want to call the point group $C_\mathrm{s}$ or $C_2$. The maths works out the same in the end — as it has to.