What it hit.
My only qualification here is to have been involved in fencing in rural Australia in my youth. What the Triplane hits is a gate post, and these are substantial and very firmly rooted in the ground. Designed to stand up to persistent leaning and shoving by cattle, they will easily win in any conflict with WW I aeroplanes; steel might have bent.
(Actually, I was involved in putting in a fence around a pistol range on a disused landfill tip. The power auger drilling holes for strainers and fence posts a couple of times broke through into an abandoned fridge.)
Very glad the pilot was OK.