I just assumed she recorded it secretly before nowTo be fair, Mihari realistically should have done this a while ago. Understanding what's actually going on physically is extremely important if she's planning on mass producing the drug. (And why wouldn't she? Plenty of people'd kill for it.) Though, realistically, she'd also need more than one test subject. So the series obviously isn't going for a 100% realistic depiction of experimentation.
after reading the first chapter of that i know understand why some people are insistent oimai is about lost trans girlhood or whatever lol, no matter how horny onimai is in comparison it's completely wholesomeJesus this one was way more lewd than the other guests, Sesese author showing everyone how it's done.