They're not really. The skill sets overlap, but neither is wholly contained in the other. The reason you see such a high crossover is these two positions tend to attract the most skillful ball players. But the roles differ.
Fullbacks need to contest high balls, tackle fast moving players often on their own, and return the ball in a high impact collision. Cross field defence and being able to read the game is critical. Being able to slot into the backline is a huge bonus. It's a challenge for short players.
Five-eighths need elite tactical kicking skills, immaculate hands, and an evasive step. They should be able to kick with both feet and pass equally well in both directions. It's one of the least demanding roles in defence with help nearby most of the time, and the offence moving slowly. However, they're often targeted by forwards aiming to hurt them. It's rare to find very tall 6's.
So basically, a taller five-eighth who's safe under the high ball and smart defender could make the switch to fullback. A fullback with exceptional kicking and ball playing skills could make the switch to five-eighth.
I could see Walsh transitioning into the halves based on his ball playing skills and football nous, but he would need to hone his kicking game. Selwyn doesn't look like a five-eighth to me at all, but he does look like a potential fullback if he can sort out his defence and fumbles.