In the city I grew up in, there’s a Chinese restaurant near my home that never gets packed, not even on busy weekends. I’ve also never seen more than 2 customers at a time eating there. And yet, it’s outlasted every single restaurant in the vicinity for 3 decades. That, and there’s always an expensive car or two parked in front.
What's the menu? Big difference between a cheap place where 2 people spend 50 between them and another place where people spend 300 between them. Also if that occupancy is all day every day and they own the building outright they could just be comfortable, especially if it's also their home.
The menu isn’t crazy expensive (for the area), but not like dirt cheap, too. You’re probably right that it might be their home so they can get by without much customers.
I got a Chinese joint two blocks away like that. I saw someone eating there for the first time for the five years that I've lived in my house. The cook is also the delivery driver, and we are both on the same one way street, so I'll see him drive past a handful of times a night. Not enough though. I don't think it's a front though. Just cheap real estate that he probably owns.