Well I really don't like exposing people, but I was gonna go out and say that senka's female voice isn't actually Senka. I first had a suspicion when i realized "he" didn't sound like any ryouseirui I heard. So what I did was do a spectrograph of his female voice, and found pretty obvious evidence. Going through points 1. The lower harmonics in the female voice are around 20db higher at the least ,even 30db, than senka's normal light voice. While a ryouseirui might have much higher 5th degree overtones, they usually aren't that high, nor the talking voice as low as senka's. 2. Usually with guy voices they tend to be peaky in higher harmonics, a pattern which tends to show even when a guy goes into a feminine tone. However in the senka female voice all those higher peaks are COMPLETELY smoothed out/attenuated. That's much more in-line with an actual female voice 3. One of the strangest changes, is that the highest harmonics in the sound wave extend in a very steady/consistent amplitude, where as with guys there are usually just random peaks in these super high harmonics. Even ryouseirui don't have this consistent high end extension, so there's almost no way that a guy could "manufacture" these extra harmonics in such a way.
Knowing all of this, it's obvious that this was sung by a female voice, and there are two more points that can solidify this. The first one being that the female voice has multiple voicing, like in world's end dancehall. This doesn't make much sense because this would even mean that senka could even fully control a female voice.. which leads into the last point, of what actual sopranists are. There are singers called sopranists who are males who have a genetic defect that didn't allow their larynx and cords to grow. I actually know a older japanese man who has this defect, and his talking voice is almost 99% indistinguishable from a mezzo-soprano talking voice. The only difference is that he has a little bit of feminine vocal fry at certain points. Otherwise he sounds nothing like a guy, but senka's normal voice is still in a normal tenor range.
Summary/tldr, Senka had one of their female friends record and sing, and then mixed that into the song without crediting them. So.. if people were wondering ever... now you know.