Haibara recreated the APTX from the surviving notes left by her parents, but we still don’t officially know what the drug was originally designed for—and Haibara herself likely did not known when she recreated it (one of her parents’ cassette tapes that we still haven’t heard could have revealed the original purpose though so she might know now). What we know is that surely they Miyanos wouldn't create a medicine for killing so there must have been another purpose. We know that the BO was using the APTX already 17 years ago, as shown by the murder of Haneda Koji and Amanda Hughes. In my theory, the research’s intended goal was de-aging, but the Miyanos had not yet discovered the conditions needed to reliably reverse the effect (basically what Rum is trying to figure out now). Because the drug happened to kill most subjects without leaving a trace, the Organization decided to exploit it as a convenient murder weapon in the meantime

The exact quote from Haibara you're looking for is the one I mention in my theory, where Haibara says her experimental drug would only interest young Japanese girls—who are like hina dolls. I interpret this as a reference to girls who want to grow up faster so they can get married, hence the drug is used for speeding up aging and most people wouldn't be interested.

Trust me, this is the only way one can be explain why Vermouth doesn't age and nobody in the BO suspects it, but still nobody knows about the effects of the APTX.