I think voice is like many things about being trans, sure females can get away with "talking like a guy" and still sound female just like they could shave their head and still look female. Passing, be it your appearance or your voice, is all about stacking up enough clues to tip the scales one way or the other. The more things we can check off, the less likely they will be to question it and dissect the details. Like you said, their voice is a bit low and a bit of resonance, if the inflection was more in the female range etc, it would tip the scales in the female direction. What first jumps out to me is they way they are talking, not "their voice".
Ad just curious too, why would we want to continue to "talk like men" when that clearly isn't who most of us strive to be? I guess it's the same reason I see so many MTF who stand,walk,sit etc like men? I never understood that either.