They are in the same way any canned good is. If you boil it, the can is likely to warp slightly and allow water in, also things like plastic liners and other chemicals can leech into your food, you generally aren’t supposed to cook food inside the cans they come in.

It’s a lower pot that you boil water in with an upper pot that you put the food in. No water gets near the food, it’s meant for applying even, indirect heat

FWIW - this picture has been floating around since the mid 2000’s; the person who blogged about it cooked it super wrong. The instructions said to use a bain marie, and they didnt know what a bain marie, but saw you boiled water in it, so they just boiled the can. If you boil a can, water is 100% going to seep into it, and turn it into…what you see here.