Yep...it is just better refined diesel.
“My understanding is that jet fuel is very much similar to, but not identical to ordinary diesel. One would assume that a chemist would be able to tell the difference.”
Correct... Jet A vs. kerosene would have key differences that would be very easy to resolve. The analysis can be performed by most commercial labs with GS-MS probably being the primary tool. Interpretation of the data would probably go from a commercial lab electronically to a specialist to interpret. From the time the sample hits the laboratory door, the results can be out in 12-24 hours with a full blown rush priority (costs more! LOL).
Back to the fingerprint though... Not only can the fingerprint ID Jet A vs. kerosene, with more time, it can track back to what refinery it was produced from, to the airport tank farm in Malaysia and perhaps even drill down to the fuel truck the delivered it to the Boeing 777. Keep shoving $$$ at the chemists and they will keep bird dogging the trail.
Oil can be analyzed much more definitively than just "diesel" "jet A" or whatever. Analysis can show source of origin of the sample, unique like a fingerprint for example.
This looks somewhat promising because it could be seepage from fuel tanks co-located near the pinger source.