It's quite plausible. However, it's also common for successful hawker stall proprietors to hire staff to do much or even all of the work for them, and since it

It's quite plausible. However, it's also common for successful hawker stall proprietors to hire staff to do much or even all of the work for them, and since it's essentially a cash-only business, it's easy to avoid taxes for a good chunk of your earnings. A good example of this is 328 Katong Laksa, once Singapore's most famous hawker stall, which has fragmented into at least half a dozen competing variants: one's at the original location, one has the license to the name, some are run by the original dude's family, etc.