Celeriac (aka: Celery Root) Skin - Edible or Not?
I too have that Jamie Oliver book (try the roast beef then make the Rendang, amazing), and you are reading the recipe correctly. There is no problem leaving the skin on providing you've washed all the soil off. In this recipe it adds to the 'zombie brain' look, but if you cooking something a bit more refined you might want to take it off so it looks better.

Also, many celeriac recipes involved mashing it, in which case taking the skin off aids that process.