TEL has a
vapor enthalpy of between 308-387K (34.85-113 deg c if I'm reading this right).
I'm no chemist, so someone with a bit more knowledge in that department might be able to go in to some more detail.
That being said, if you look at the video you'll see them siphoning fuel by sucking on a hose, so any fluid or fluid particles entering the airway or mouth will contain TEL regardless of temperature. TEL can also enter the body via the skin, so that's just another way the levels could be shown to be higher in people sniffing avgas.