It is true that fuel is so difficult to ignite at altitude.
Aaaamen. Re the two flashes reported by US, supposedly one at altitude and one at/near ground- apparently by using Infra-red technology. It **MAY** be that as plane tumbled as a result of loss of tail- HS , what was seen was looking essentially into the aft end of the two jet engines still running or still hot, making a great IR target.
Then of course when hitting the ground would be relatively easy to ignite spilled fuel giving the second IR flash.
Could there be " readable " flashes between air and ground ? Maybe- depending on resolution and tumbling of wings and partial body - but that degree of sensitivity would likely NOT be public information.
