"Mixed venous oxygen saturation has been shown to be a surrogate for the cardiac index as a target for hemodynamic therapy ".
He references this paper which in turn references this. Maybe looking another layer back through the references would provide the answer. This paper looked promising, but actually the magical percentage was discovered by earlier researchers.
So here it is. The paper giving the evidence for the value quoted in the surviving sepsis guideline comes from a coronary care unit in 1968.
Clearly not everything we do can be based on a meta-analysis of double blind RCTs conducted within the last 6 months, but things are often stated as fact - it can be useful to track down the reference (but life's too short not to take some things at face value!).