They actually don't, they don't even give a mention at all one of the RS Gold biggest impacts on inflation: players leaving the game and their accounts 'hoarding' all this money which will no longer enter circulation.
That's called the Miser Effect, and it is effectively deflation. All other money chases the same goods without competition from this money. Therefore it effectively doesn't exist after a period of time.
Take the Adams Family. Presumably at one point they actually interacted with the outside world to make their fortune in the first place and to have the house built. Then they go crazy and hoard all their money in the basement...dungeons.
For a while after that point, the world assume the money exists as functional purchasing power as it had been in circulation just hours/days/weeks/months before. Prices are not perfectly elastic, so it will take a while before the fortune is out of the economy totally.
The process that lead to this system being developed was, ironically enough, one of the largest bot developers turned over their code to Jagex and then helped them write and design a series of anti-bot measures.