Hardnut

Pentti Airikkala is a former British, Finnish and Scandinavian Rally Champion, and what he doesn't know about throwing a car through a forest you may as well not bother with.

what does the sentence mean?

and
Spend a weekend learning to skin rabbits, make nettle soup and erect shelters made of bin liners in the capable company of hardnut ex-SAS instructors

Hardnut?

"Hardnut," I should think, means tough, macho, no-nonsense, stoic, energetic.

Don't know about throwing a car through a forest except that the general impression is that the man is an expert at car racing.