Sloan also went from 84kg to 93kg. I doubt that we will see him playing lock next year. NRL footballers always get as big as they can bcs it is the only way to protect yourself against collisions.He won't be keeping that weight gain once he starts playing. He will trim back substantially but leaving additional strength.
My theory is that he has done this to improve his strength against a further Creighton-type smother tackle.
As for keeping the muscle on through a long season, as you point that is really difficult too. That is why many footballers no longer drink alcohol, bcs it is catabolic (it strips off muscle.)