Not an expert - but I'd think that ancient people (at least in non-arid climates) would have noticed that the more-charred part of the wood in an old campfire did not crumble and rot, even over years.
Agreed, maybe a forest fire or lightning strike might have made it more obvious that charring preserved. Charcoal in a campfire could have easily disappeared in the next fire.
They do give Leonardo a lot of credit for a single sentence, but it may be the first documented instance of this charring technique.