Comment by defrost

They specifically mentioned "grasslands" which covers a range of not forrest type lands, from waist high dense grass to sparser knee high grass bush land.

It's common enough for indigenous people to burn off dry grass ranges every year or two, often in chequered patterns to lessen the chances of wind picking up and fanning a full front across unburnt grasslands.

That's likely the 10% referred to, repeated burning of grasslands along with the livable fringes and common paths of forrest areas.

Add onto that "natural" fires from lightening strikes, etc. Some of these would start in ares with little human management and years of built up leaf litter leading to big burns that reduce large areas to ash on the ground and a few scattered trunks .. many would start in areas that have had fires in past five years or so and would result in "cool burns" through leaf litter, some tree trunk climbing, but essentially leave big trees standing and alive with clear floors for new growth.