Accurate quantum simulations of the low-temperature inelastic neutron scattering (INS) spectra of HF@C are reported, for two incident neutron wavelengths. They are distinguished by the rigorous inclusion of symmetry-breaking effects in the treatment, and having the spectra computed with HF as the guest, rather than H or HD, as in the past work. The results demonstrate that the precedent-setting INS selection rule, originally derived for H and HD in near-spherical nanocavities, applies also to HF@C, despite the large mass asymmetry of HF and the strongly mixed character of its translation-rotation eigenstates. This lends crucial support to the theoretical prediction made earlier that the INS selection rule is valid for any diatomic molecule in near-spherical nanoconfinement. The selection rule remains valid in the presence of symmetry breaking, but is modified slightly in an interesting way. Comparison is made with the recently published experimental INS spectrum of HF@C. The agreement is very good, apart from one peak for which our calculations suggest a reassignment. This reassignment is consistent with the measured INS spectrum presented in this work, that covers an extended energy range.