He was found near to his home, so he may have ignited himself accidentally there, on the cooker or fire, then ran into the street for help. Dry clothes will burn readily without an accelerant.
The Phenomena of spontaneous combustion is not what is described in the story. It is usually a slow burn of body fat when the body is incapacitated or dead.
There are several non-fiction books on cases of spontaneous human combustion, some of which include very convincing photographs, which I have read. I have an open mind on the subject.
///Spontaneous human combustion (SHC) is a term encompassing reported cases of the combustion of a living (or very recently deceased) human body without an apparent external source of ignition///
There are a very few cases where researchers cannot explain the fire which consumes a person. However, these researchers have noted that in every one of those cases which might be spontaneous combustion, alcohol is involved. It remains to be seen if any other explanation emerges in this case.
I remember QED doing an episode on Spontaneous Human Combustion, many many years ago. It might be worth a rewatch if it is around on YouTube or somewhere. Gromit's theory seems likely. though.
Dickens wrote about a case of
S.H.C.in, I think - open to correction -'Bleak House'. I gather there had been an inexplicable occurrence of which he was aware. I'll be very interested to see what forensics come up with.
I don't know if that's what caused his burning but I don't think anyone can dismiss spontaneous combustion as a non-existent phenomenon. It's what ABers in the 16 century might have said about flying, phones, TV, space travel etc etc etc.