I mean...
Keiko Han did not make Luna sound as young as some people around here like to claim. She definitely sounds like a woman in her 20s and not a girl a year younger than Usagi. Yet, in PGSM, she turns into a girl who looks like she's about 10. So...... I know that not everyone likes Sailor Luna, but if Naoko and Toei had no problem with adult woman-sounding plush toy Luna turning into a little girl, I don't get why anyone should think anything of Jill Frappier's Queen Victoria meets C3-PO Luna assuming the human form she assumes in the S movie (especially since one can easily infer that she just uses Christmas/Silver Crystal magic to assume the form of Princess Kaguya, and not her intrinsic human form).
Still... Since Tracey Moore was originally going to audition for Luna, presumably at first they did plan to give Luna a voice closer to Keiko's. I wonder when/why they changed direction? (Maybe after casting Tracey they wanted to make sure Luna had a more distinctive and authoritative voice?)