Even without a second witness, police get warrants for arrest routinely with just a signed affidavit, which is just a formal accusation. They don't even need that if they feel there is danger involved to either party. In a case like this, however, all there needs to be as "more than reasonable suspicion" that a crime occurred to invoke probable cause. The case law on this is murky because SCOTUS has intentionally left it to the lower courts to define, and they have been scattershot in their findings. This is why so many people are championing police reforms. In reality, a policeman gets to determine probable cause at the scene, and in this case, a judge had to determine there was probable cause, which is a very low bar legally.
Now, as for specifics to this case, I have no inside info, but I am frankly disturbed that a judge issued a warrant for arrest considering the crime was not recent. Unless there was a rape kit performed at the time, but the rape was not reported, I don't see how there could be evidence to support, excepting video or witnesses. That being said, procedurally, the arrest may have happened because Gotsis is not a US citizen and therefore could be used to bar him from international flights pending results of an investigation. (This is pure speculation on my part as well)