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.