They took off with center tank empty. Nothing but very volatile fumes.
Filling or partially filling tanks was a safety measure I belive that was adopted.
You wrote: “...They took off with center tank empty. Nothing but very volatile fumes.”
Here is the Jet A1 chemical composition: “..In summary, the composition of Jet A/A-1, JP-5, and JP-8 are very similar. They consist predominantly of C9–C16 hydrocarbons that are a combination of n-paraffins, isoparaffins, naphthenes, and aromatics”.
Boiling point of C9 paraffins is ~150oC, C12 ~219oC and C16 is 286oC. The outside temperature on the ground on that fateful day was around 30oC. The plane came from Paris in the afternoon and its fuel temperature (when they landed) was probably way below freezing point, as the air temperature at the cruising altitude is -50oC. The plane sat on the tarmac for a few hours and took off around 6 pm. There was no way in hell the temperature of the fuel in that fuel tank was high enough to produce enough vapors even if the ignition source was somehow present. And aviation fuel, and its vapors, do burn, but do not blow up.