Consider what this article says:
The primary concern for shoppers is other people, not food. Though shoppers might worry about infecting themselves by handling the same apple or Cheerios box as someone else, health experts say transmission through food or its wrapping is largely avoidable. Research suggests that the virus can exist on cardboard food packaging for a day, and on plastic for several days, but it becomes less infectious over these periods. “My recommendation is just to wash your hands after you handle external packaging,” Angela Rasmussen, a virologist at Columbia University, says. “High-touch” objects such as shopping-cart handles are a bigger concern, but many stores have provided sanitizing wipes for those.
So research suggests that the virus can exist on cardboard food packaging for a day, and on plastic for several days. We are "being told" this by articles like the above, but not by the authorities as you correctly say. I also agree with your comment "how far is too far", so you just have to take a few risks with anything in life. Getting food during the lockdown is possibly the biggest risk area that we face, so some care is obviously needed.
I personally feel its worth santizing the packaging. You go shopping, dont bother, just pack it away, wash hands. Make a peanut better sandwich, need to wash hands before eating. Forgot to put it away? Put it away, wash hands. Cornflakes packet tip over, fix that, wash hands. You'd need to either leave it all in the garage for 3 days , or accept that there is potentially virus on most packages. Its catchable from surfaces, to hands, to mouth, eyes, and nose. High touch surfaces are a bigger concern, so she has stated that low touch is a concern.