"powered by anti-gravitic engines", it doesn't actually say what the fuel source is.
Burning fuels - unlikely, the compact nature of the jetbikes and the amount of energy that would be required to create the anti-grav field would be rather large. You would need a lot of hydrogen to create that much energy.. this of course assumes our laws of physics apply to 40k..
Electric- possibly, they probably solved the storage/generation issues, so this would be the most likely except...
All Eldar craft seem to have Jet engines and air intakes.. but they also need to work in space with no atmosphere... so maybe they are hybrids lol.