Hi, welcome to GoY, that only applies to the lemons that were on the plant when you bought it, any fruits that you get from flowers that came afterwards can be used, Derek.

I would disregard the warning. The only reason that they say this is to to cover themselves under the ever increasing food regulations mostly the ones that the EU thrust upon us. Garden centres sell them as plants, not food. If they were implying that they were to be eaten, then they would come under food regulations and they would have to be inspected accordingly, a big headache.

Enjoy your lemons, you won't die, they are totally safe to eat.