I would think that they are referring to the sushi that you buy from a food retailer that is mass produced and could sit all day in a bain marie. Firstly you don't know how fresh the raw ingredients are and secondly would you be confident that their fridges are working effectively enough to keep it at the proper temperature which I believe should be 4 degrees.

I make my own with thin well cooked honey-soy marinated chicken strips with capsicum, cucumber and avocado
If you have prepared the whole lot including the chicken and the rice and are confident that your food handling practices are safe then go ahead and enjoy it.

I would steer clear of adding anything like cold precooked prawns or raw seafood at all to be on the safe side.