I think the parent comment was talking about a loyalty card discount at a supermarket that (I assume) only exists in the UK and Ireland.
They are & so is the parent of your post. In the US, it's common for supermarket cashiers to lookup loyalty cards by the customer's phone number. The comment you replied to is saying that the phone number <area-code>-867-5309 is almost always tied to an existing loyalty card. Lots of people just give that phone number if they want the discounts without signing up.
You can also get a copy of a barcode by various nefarious means, if needed.