I think there are definitely a few of them that are trans, most notably Lisa, but to say all of them are trans seems too far fetched. You can see photos of some of them as kids. Or i just dont wanna believe thisYou have a great point however I think that voice acting and singing is a lot different and latter requires more effort especially if it is high notes. In my office we have a few tr@ns. Most of them when they try to speak feminine, you can feel that they are faking their voice . In addition to that, we have one that can speak very feminine without exerting any effort, however that person cannot sing very well in femine voice.
If you think about it , idols sing for like 3 hours in a concert, that would be very tiring and perhaps some of them would slip up to show their true vocals because of fatigue. Overall I think you bring up a sensible point.