It is also somewhat misleading to say they took off “without fuel” if they took off with charged batteries, as electricity is the fuel of electric motors.
That said, it’s quite possible they took off with discharged batteries, and will charge them for the night flight during the previous day, but I doubt it.
It is my understanding that when they race those solar cars in Australia, that is exactly what they do. The batteries must be charged by the sun for some time prior to the race.
I think that is really a non-issue. If they have the ability to fully charge the batteries from sitting in sunlight, then what difference does it make if they do it that way versus charging them up the other way?