the ladder isn't necessarily nailed to the A-frame. It would still be stable without nails, it can't slide to the sides, and presumably the beginning of the ladder is held in place somehow, so the ladder won't slip backwards off the base either.
i would never build something of this height with that width of board( which appears to be 2x4? 4x1? it's thin whatever it is). i would not use it on the columns/Aframe, not on the stringers, i don't even like it as decking. i suppose it works since this guy is going off it, and i see the lateral supports built into the columns, but its really a wooden stunt of low quality.i suppose this sounds rude of me, and i don't intend to argue about it, but this should not be a stunt worth pondering the manner of build. feel free to look in my photos under guerilla bastards for my approach and methods of trailbuilding.