Either they've completely documented the stack and API so that new employees can get up to speed quickly and if their lead developers get hit by a bus they can

Either they've completely documented the stack and API so that new employees can get up to speed quickly and if their lead developers get hit by a bus they can continue as a viable business. In which case they've spent lots of valuable resources working on documentation. Or their technology is poorly documented and basically an oral tradition. God help them if their lead developers get hit by a bus.