I think that would be a particularly heartless interpretation of the rules ;-)
This is the photo
The gray fence is "forbidden to cross" and the black fence is not mapped. In the rules there is no definition that I can find about what "crossing" is - if someone's head crosses the feature is that forbidden?? At the French WOC Sprint final a number of years ago I think they ruled that you can't put a foot in a forbidden area feature, but other parts of your body could be above it. But I don't know any precedent for what constitutes crossing a line feature.
"Crossing" is an interesting word because you could jump across a feature and not disturb it which is clearly "crossing". But you could also reach across (eg: to punch a control on the other side of an uncrossable feature) without disturbing the feature (eg: flower bed, or fence) - would that violate the rule since only your hand (and SI stick) crossed the feature.
In this case though any official complaint against the runner would I think have to be thrown out regardless since in ISSOM the rule is:
The course planner should not encourage unfair actions from the competitors, such as crossing barriers or areas with forbidden access. If it is unavoidable to set legs that cross or skirt areas with forbidden access or impassable walls and fences, then they have to be marked in the terrain, and observers should be present at the critical points.
In this case the route choice was to run to the end of the forbidden-to-cross fence (the grey one) and cross there. But the other fence (the black one) was not mapped, so it was not clear where the forbidden area ended and so it should have been marked (if you intend to DQ people for crossing it). In fact some people were so cautious about not crossing a forbidden feature that the ran all the way to the end of the unmapped fence to be sure - something is wrong in that I think.