A mod 9 generating-function congruence for the Gordon–McIntosh mock theta coefficients

Let $p_{\xi}(n)$ denote the coefficients of the Gordon–McIntosh mock theta function $\xi(q)$, so that $\xi(q)=\sum_{n\geq 0}p_{\xi}(n)q^n$. Generating-function congruence. One has

$$\sum_{n=0}^\infty p_{\xi}(32n+12)q^n \equiv 6\sum_{n=0}^\infty q^{3n(n+1)/2} \pmod{9}.$$

This is the second of two conjectured congruences motivated by computational evidence for the coefficients of $\xi(q)$; its status is not resolved in the source.