This is rather surprising, taking into consideration how earlier in the middle of this year, we did mention that iOS 9 might be nearly impossible to jailbreak. I guess the person did not get the memo, and must be a member of the Impossible Missions Force. Apple iOS hacker iH8snow published a video as seen above, where it depicts an untethered jailbreak for an iPhone 5 that was running a pre-release build of iOS 9. iH8show also claimed that this method works with iOS 9.1 beta, too.
Impossible is indeed nothing. Do bear in mind that Apple does not support jailbreaking an iPhone or iPad officially, so the only way to do so is to sniff out a security vulnerability and exploit that loophole in order to have the device run unsigned code. It remains to be seen how long this method will last before Apple patches it up.