He was perhaps "having you on"
Coke production is more industrial (and isn't made like that), nor a cottage industry like you saw there.
BTW, have you seen it more than once?, I would suspect it's only a very occasional process?.
It has to come from somewhere
(although I also doubt it's the Derbyshire Dales) - I would imagine it was being done to use the wood cut down from clearing the paths and grassland, making a saleable product whereas small parts of trees and bushs wouldn't be worth the cost of removal.
Perhaps you weren't aware?, but before the advent of coal in iron making, charcoal was used instead (the invention of using coal instead was made in Ironbridge, Shropshire), and this devastated huge areas of forestland in the UK where charcol was produced, just as you saw in Lathkill Dale.
No coal mines
those were lead mines, the area is covered with them - many totally unknown - this is particularly dangerous in many areas of Derbyshire, because many shafts were 'capped' simply by placing planks across them. Over the many decades (centuries?) the soil and grass grows over the top completely hiding them, and the planks rot away - leaving deep shafts covered by nothing but a layer of grass and an inch or two of soil.
Aren't the Moors nice there?, I grew up on the edge of Stanton Moor, at a small village called Birchover.