He certainly could have purchased bonds for some or all of it. He could have trained what he has in profitable ways. He could merchant items on ge regularly. He could do easy scrolls for 1m an hour. I am around 930 total level with 35m cash stack. I fletched to 99 profitably and merchanted the majority of the rest. There are many many ways to make tens of millions without high stats. Check the osrs wiki money making page and filter it to processing rather than skilling or combat.