No family? Also if we were talking about the US (which the article was) then you have to factor in healthcare, average of $200 a month for the insurance IF it's sponsored (mostly covered) by their employer. If the grocery worker is living in any major city then rent is bare minimum of $600 a month for the privilege of living with at least 2 other flatmates or around $1000 for a pretty small apartment. So this person's already out half their monthly income. Let's say transportation costs are low and that this person only takes the bus to get everywhere, that's an extra $100 a month (assuming lack of adequate transportation doesn't also ending up costing them their job due to frequent late starts). Now this person also has to pay at least some utilities depending on where they're living so a rough estimate would be around $120 a month for that. Now we look at food, average grocery costs per person in the US is $220 a month. This person also needs clothing obviously, if they go on the super low side they could get by on ~$100 a month over all. If this is it for their spartan life style, meaning no cell phone, no internet, no eating out, no leisure activities that cost money, no hobbies, no children, no trips to the doctor (where medical deductibles kick in), no car, no netflix etc. This person has potentially $460 a month to save.
That's 100% best case scenario living a lifestyle that basically consists of going to work, going home, eating food, sleeping on the floor of their apartment (we didn't budget for furnishings but w/e it's better for the back) using the public library and walks around the park for sole avenues of entertainment. ALSO: THERE'S NO KIDS IN THIS SCENARIO. Some of those estimates I lowballed also (like food as that's average cost of groceries for people who also get calories from eating out at restaurants) so this is really not a realistic estimate here.
In reality though there's plenty of hidden costs that this person will incur and even assuming they do manage to stay kidsless they'll be lucky to put away $100 a month giving them a best case scenario of $250k in savings by 55. Not terrible for them, but not enough to retire on in the US when you think of medical expenses incurred later on in life.