If theyre working as quants now, they were probably around before RS3 as well. I remember daytrading the grand exchange personally. There were plenty of profitable patterns that were basically guaranteed to work. I can see how someone who could program at the time could get much higher volumes by automating multiple accounts. The problem was finding profitable new trades at different levels of wealth, what worked at one volume would have to be replaced eventually. I never got into clans and such. I got scammed a few times especially memorable was a scam with a sort of party of social proof around the scammer and they would rinse repeat in every world, some things were so organised that you got really surprised by the intricacy of the scams.

If you were one of the rubes falling for Smoking Mils sure. But there was always plenty of money to be made from placing stink orders for high volume items though, to name one very basic strategy. When you have a big enough quantity for an in-demand, relatively unvolatile asset like prayer pots you can essentially just have orders in where you get paid by the spread from bigger orders and be the main market maker. There wasn't exactly a CLOB but it was pretty easy to figure out the spread by buying 100 then selling it back for max/min price.

Other players would treat the GE like an NPC where they'd dump items in mass at 1gp just to get it all sold - knowing and taking advantage of this was (and prob still is) free money.

There were some very fun strategies where like, you would submit a question for the QnA proposing some use for $item around high alch price (e.g. 'the dark bow used to be such an iconic weapon but now it's useless and feels pathetic selling for 70k. with the new boss coming out, could we see a new use for these? :((') and when it got read out and if they liked the idea, speculators would buy these in bulk and especially if they weren't a very liquid item you would cash in, with minimal downside if you don't get read. Jmods caught on to this though, lol.

I think the natural scepticism it teaches makes for a good quant too. I remember seeing a Twitter thread like, 'If I have a coin and show you the two sides, then flip 19 heads in a row, what price should you accept to bet that it would be heads again?' with the options all being well >50%. The comments were all like, 'Well based on Laplace's rule of succession...', '...based on a beta distribution with a uniform prior...', etc., all theoretically correct stuff if we live in a mathematically perfect world.

The Runescape-brained would give you the real-life most likely answer: 'That coin is rigged, dude. I'm going to find better value elsewhere.'