On its face, the judgment sounds nuts.
From what we can see in the media reporting, the factors supporting reasonable doubt were that the glass the girl drank from did not show any trace of the drug, the absence of DNA evidence and that it wouldn't make much sense for him to first take her back to his apartment before then taking her to a very busy, public place to assault her. Maybe that's enough for "reasonable doubt".
Perhaps there's other relevant evidence too, like his search history.
That said, his conduct is completely unfathomable. The story as to how she ended up back at his apartment might not be completely lacking in credibility but keeping her there for 2 hours, without contacting anyone? That in itself may not be evidence of a crime but I just do not see how there could be any remotely credible explanation for it, other than nefarious ones.