I believe he was citing this study on the temp differences in viral survival http://www.mdpi.com/1999-415/4/10/2115/pdf
No one has addressed the “when” in the course of the disease, is the most infective stage.
Most of those returning from Africa were treating very ill patients, those close to death. Would the likely hood of airborne transmittal be greater at this time?
Compare to someone infected who is just now coming down with some symptoms. There seems to be no one infected from Duncan’s early stages, but rather the later stages.