Interestingly we don't have any hard evidences either way. Simply guessing ( educated one may add) .
The fact the arsonist wasn't always catch up and then sentenced doesn't mean anything.
We must agree the smart arsonist may rather win alway over prosecutor.
We don't have also hard evidenced of bushfires ignited by lighting.
This is rather more based circumstances - there was lighting, then bushfire, then lighting was at fault- but nothing like video security camera or taken from drones , space video of single lighting strike spreading into massive bushfire.
not very convincing evidence. tree is not instantly erupting into torch, but one may say that our trees are different and lighting strikes also 1000x stronger.
But I would like to see a single photo / video evidence from our recent bushfire of such event. One video surveillance demonstrating cause of one of many recent bushfire here.